Prefer const/let over var

42 files changed, 1094 insertions, 1134 deletions

diff --git a/.eslintrc b/.eslintrc
index b65b16c..aef4c0d 100644
--- a/.eslintrc
+++ b/.eslintrc
@@ -9,6 +9,7 @@
   "extends": "eslint:recommended",
   "rules": {
       "no-unused-vars": ["error", { "vars": "all", "args": "none", "ignoreRestSiblings": true }],
-      "no-constant-condition": ["error", { "checkLoops": false }]
+      "no-constant-condition": ["error", { "checkLoops": false }],
+      "no-var": "error"
   }
 }
diff --git a/app/error-handler.js b/app/error-handler.js
index 5a2650f..d813349 100644
--- a/app/error-handler.js
+++ b/app/error-handler.js
@@ -2,21 +2,20 @@
 // native support in the browsers, so that our error handler
 // can catch script-loading errors.
 
-
 (function(){
     "use strict";
 
     // Fallback for all uncought errors
     function handleError (event, err) {
         try {
-            var msg = document.getElementById('noVNC_fallback_errormsg');
+            const msg = document.getElementById('noVNC_fallback_errormsg');
 
             // Only show the initial error
             if (msg.hasChildNodes()) {
                 return false;
             }
 
-            var div = document.createElement("div");
+            let div = document.createElement("div");
             div.classList.add('noVNC_message');
             div.appendChild(document.createTextNode(event.message));
             msg.appendChild(div);
@@ -24,7 +23,7 @@
             if (event.filename) {
                 div = document.createElement("div");
                 div.className = 'noVNC_location';
-                var text = event.filename;
+                let text = event.filename;
                 if (event.lineno !== undefined) {
                     text += ":" + event.lineno;
                     if (event.colno !== undefined) {
diff --git a/app/localization.js b/app/localization.js
index 43c1294..11144f7 100644
--- a/app/localization.js
+++ b/app/localization.js
@@ -21,25 +21,24 @@ export function Localizer() {
 Localizer.prototype = {
     // Configure suitable language based on user preferences
     setup: function (supportedLanguages) {
-        var userLanguages;
-
         this.language = 'en'; // Default: US English
 
         /*
          * Navigator.languages only available in Chrome (32+) and FireFox (32+)
          * Fall back to navigator.language for other browsers
          */
+        let userLanguages;
         if (typeof window.navigator.languages == 'object') {
             userLanguages = window.navigator.languages;
         } else {
             userLanguages = [navigator.language || navigator.userLanguage];
         }
 
-        for (var i = 0;i < userLanguages.length;i++) {
-            var userLang = userLanguages[i];
-            userLang = userLang.toLowerCase();
-            userLang = userLang.replace("_", "-");
-            userLang = userLang.split("-");
+        for (let i = 0;i < userLanguages.length;i++) {
+            const userLang = userLanguages[i]
+                .toLowerCase()
+                .replace("_", "-")
+                .split("-");
 
             // Built-in default?
             if ((userLang[0] === 'en') &&
@@ -48,12 +47,11 @@ Localizer.prototype = {
             }
 
             // First pass: perfect match
-            var j;
-            for (j = 0; j < supportedLanguages.length; j++) {
-                var supLang = supportedLanguages[j];
-                supLang = supLang.toLowerCase();
-                supLang = supLang.replace("_", "-");
-                supLang = supLang.split("-");
+            for (let j = 0; j < supportedLanguages.length; j++) {
+                const supLang = supportedLanguages[j]
+                    .toLowerCase()
+                    .replace("_", "-")
+                    .split("-");
 
                 if (userLang[0] !== supLang[0])
                     continue;
@@ -65,11 +63,11 @@ Localizer.prototype = {
             }
 
             // Second pass: fallback
-            for (j = 0;j < supportedLanguages.length;j++) {
-                supLang = supportedLanguages[j];
-                supLang = supLang.toLowerCase();
-                supLang = supLang.replace("_", "-");
-                supLang = supLang.split("-");
+            for (let j = 0;j < supportedLanguages.length;j++) {
+                const supLang = supportedLanguages[j]
+                    .toLowerCase()
+                    .replace("_", "-")
+                    .split("-");
 
                 if (userLang[0] !== supLang[0])
                     continue;
@@ -94,21 +92,19 @@ Localizer.prototype = {
     // Traverses the DOM and translates relevant fields
     // See https://html.spec.whatwg.org/multipage/dom.html#attr-translate
     translateDOM: function () {
-        var self = this;
+        const self = this;
         function process(elem, enabled) {
             function isAnyOf(searchElement, items) {
                 return items.indexOf(searchElement) !== -1;
             }
 
             function translateAttribute(elem, attr) {
-                var str = elem.getAttribute(attr);
-                str = self.get(str);
+                const str = self.get(elem.getAttribute(attr));
                 elem.setAttribute(attr, str);
             }
 
             function translateTextNode(node) {
-                var str = node.data.trim();
-                str = self.get(str);
+                const str = self.get(node.data.trim());
                 node.data = str;
             }
 
@@ -153,8 +149,8 @@ Localizer.prototype = {
                 }
             }
 
-            for (var i = 0;i < elem.childNodes.length;i++) {
-                var node = elem.childNodes[i];
+            for (let i = 0; i < elem.childNodes.length; i++) {
+                const node = elem.childNodes[i];
                 if (node.nodeType === node.ELEMENT_NODE) {
                     process(node, enabled);
                 } else if (node.nodeType === node.TEXT_NODE && enabled) {
@@ -167,5 +163,5 @@ Localizer.prototype = {
     },
 };
 
-export var l10n = new Localizer();
+export const l10n = new Localizer();
 export default l10n.get.bind(l10n);
diff --git a/app/ui.js b/app/ui.js
index 8515387..d587cfc 100644
--- a/app/ui.js
+++ b/app/ui.js
@@ -18,7 +18,7 @@ import Keyboard from "../core/input/keyboard.js";
 import RFB from "../core/rfb.js";
 import * as WebUtil from "./webutil.js";
 
-var UI = {
+const UI = {
 
     connected: false,
     desktopName: "",
@@ -101,7 +101,7 @@ var UI = {
 
         document.documentElement.classList.remove("noVNC_loading");
 
-        var autoconnect = WebUtil.getConfigVar('autoconnect', false);
+        let autoconnect = WebUtil.getConfigVar('autoconnect', false);
         if (autoconnect === 'true' || autoconnect == '1') {
             autoconnect = true;
             UI.connect();
@@ -131,12 +131,10 @@ var UI = {
     },
 
     initSettings: function() {
-        var i;
-
         // Logging selection dropdown
-        var llevels = ['error', 'warn', 'info', 'debug'];
-        for (i = 0; i < llevels.length; i += 1) {
-            UI.addOption(document.getElementById('noVNC_setting_logging'),llevels[i], llevels[i]);
+        const llevels = ['error', 'warn', 'info', 'debug'];
+        for (let i = 0; i < llevels.length; i += 1) {
+            UI.addOption(document.getElementById('noVNC_setting_logging'), llevels[i], llevels[i]);
         }
 
         // Settings with immediate effects
@@ -145,7 +143,7 @@ var UI = {
 
         // if port == 80 (or 443) then it won't be present and should be
         // set manually
-        var port = window.location.port;
+        let port = window.location.port;
         if (!port) {
             if (window.location.protocol.substring(0,5) == 'https') {
                 port = 443;
@@ -172,16 +170,16 @@ var UI = {
     },
     // Adds a link to the label elements on the corresponding input elements
     setupSettingLabels: function() {
-        var labels = document.getElementsByTagName('LABEL');
-        for (var i = 0; i < labels.length; i++) {
-            var htmlFor = labels[i].htmlFor;
+        const labels = document.getElementsByTagName('LABEL');
+        for (let i = 0; i < labels.length; i++) {
+            const htmlFor = labels[i].htmlFor;
             if (htmlFor != '') {
-                var elem = document.getElementById(htmlFor);
+                const elem = document.getElementById(htmlFor);
                 if (elem) elem.label = labels[i];
             } else {
                 // If 'for' isn't set, use the first input element child
-                var children = labels[i].children;
-                for (var j = 0; j < children.length; j++) {
+                const children = labels[i].children;
+                for (let j = 0; j < children.length; j++) {
                     if (children[j].form !== undefined) {
                         children[j].label = labels[i];
                         break;
@@ -224,8 +222,8 @@ var UI = {
         // resize events aren't available for elements
         window.addEventListener('resize', UI.updateControlbarHandle);
 
-        var exps = document.getElementsByClassName("noVNC_expander");
-        for (var i = 0;i < exps.length;i++) {
+        const exps = document.getElementsByClassName("noVNC_expander");
+        for (let i = 0;i < exps.length;i++) {
             exps[i].addEventListener('click', UI.toggleExpander);
         }
     },
@@ -329,7 +327,7 @@ var UI = {
     // Add a call to save settings when the element changes,
     // unless the optional parameter changeFunc is used instead.
     addSettingChangeHandler: function(name, changeFunc) {
-        var settingElem = document.getElementById("noVNC_setting_" + name);
+        const settingElem = document.getElementById("noVNC_setting_" + name);
         if (changeFunc === undefined) {
             changeFunc = function () { UI.saveSetting(name); };
         }
@@ -383,7 +381,7 @@ var UI = {
         document.documentElement.classList.remove("noVNC_disconnecting");
         document.documentElement.classList.remove("noVNC_reconnecting");
 
-        let transition_elem = document.getElementById("noVNC_transition_text");
+        const transition_elem = document.getElementById("noVNC_transition_text");
         switch (state) {
             case 'init':
                 break;
@@ -444,7 +442,7 @@ var UI = {
     },
 
     showStatus: function(text, status_type, time) {
-        var statusElem = document.getElementById('noVNC_status');
+        const statusElem = document.getElementById('noVNC_status');
 
         clearTimeout(UI.statusTimeout);
 
@@ -550,15 +548,15 @@ var UI = {
     toggleControlbarSide: function () {
         // Temporarily disable animation, if bar is displayed, to avoid weird
         // movement. The transitionend-event will not fire when display=none.
-        var bar = document.getElementById('noVNC_control_bar');
-        var barDisplayStyle = window.getComputedStyle(bar).display;
+        const bar = document.getElementById('noVNC_control_bar');
+        const barDisplayStyle = window.getComputedStyle(bar).display;
         if (barDisplayStyle !== 'none') {
             bar.style.transitionDuration = '0s';
             bar.addEventListener('transitionend', function () {
                 this.style.transitionDuration = ""; });
         }
 
-        var anchor = document.getElementById('noVNC_control_bar_anchor');
+        const anchor = document.getElementById('noVNC_control_bar_anchor');
         if (anchor.classList.contains("noVNC_right")) {
             WebUtil.writeSetting('controlbar_pos', 'left');
             anchor.classList.remove("noVNC_right");
@@ -572,7 +570,7 @@ var UI = {
     },
 
     showControlbarHint: function (show) {
-        var hint = document.getElementById('noVNC_control_bar_hint');
+        const hint = document.getElementById('noVNC_control_bar_hint');
         if (show) {
             hint.classList.add("noVNC_active");
         } else {
@@ -583,9 +581,9 @@ var UI = {
     dragControlbarHandle: function (e) {
         if (!UI.controlbarGrabbed) return;
 
-        var ptr = getPointerEvent(e);
+        const ptr = getPointerEvent(e);
 
-        var anchor = document.getElementById('noVNC_control_bar_anchor');
+        const anchor = document.getElementById('noVNC_control_bar_anchor');
         if (ptr.clientX < (window.innerWidth * 0.1)) {
             if (anchor.classList.contains("noVNC_right")) {
                 UI.toggleControlbarSide();
@@ -599,15 +597,15 @@ var UI = {
         if (!UI.controlbarDrag) {
             // The goal is to trigger on a certain physical width, the
             // devicePixelRatio brings us a bit closer but is not optimal.
-            var dragThreshold = 10 * (window.devicePixelRatio || 1);
-            var dragDistance = Math.abs(ptr.clientY - UI.controlbarMouseDownClientY);
+            const dragThreshold = 10 * (window.devicePixelRatio || 1);
+            const dragDistance = Math.abs(ptr.clientY - UI.controlbarMouseDownClientY);
 
             if (dragDistance < dragThreshold) return;
 
             UI.controlbarDrag = true;
         }
 
-        var eventY = ptr.clientY - UI.controlbarMouseDownOffsetY;
+        const eventY = ptr.clientY - UI.controlbarMouseDownOffsetY;
 
         UI.moveControlbarHandle(eventY);
 
@@ -619,18 +617,18 @@ var UI = {
 
     // Move the handle but don't allow any position outside the bounds
     moveControlbarHandle: function (viewportRelativeY) {
-        var handle = document.getElementById("noVNC_control_bar_handle");
-        var handleHeight = handle.getBoundingClientRect().height;
-        var controlbarBounds = document.getElementById("noVNC_control_bar")
+        const handle = document.getElementById("noVNC_control_bar_handle");
+        const handleHeight = handle.getBoundingClientRect().height;
+        const controlbarBounds = document.getElementById("noVNC_control_bar")
             .getBoundingClientRect();
-        var margin = 10;
+        const margin = 10;
 
         // These heights need to be non-zero for the below logic to work
         if (handleHeight === 0 || controlbarBounds.height === 0) {
             return;
         }
 
-        var newY = viewportRelativeY;
+        let newY = viewportRelativeY;
 
         // Check if the coordinates are outside the control bar
         if (newY < controlbarBounds.top + margin) {
@@ -651,15 +649,15 @@ var UI = {
         }
 
         // The transform needs coordinates that are relative to the parent
-        var parentRelativeY = newY - controlbarBounds.top;
+        const parentRelativeY = newY - controlbarBounds.top;
         handle.style.transform = "translateY(" + parentRelativeY + "px)";
     },
 
     updateControlbarHandle: function () {
         // Since the control bar is fixed on the viewport and not the page,
         // the move function expects coordinates relative the the viewport.
-        var handle = document.getElementById("noVNC_control_bar_handle");
-        var handleBounds = handle.getBoundingClientRect();
+        const handle = document.getElementById("noVNC_control_bar_handle");
+        const handleBounds = handle.getBoundingClientRect();
         UI.moveControlbarHandle(handleBounds.top);
     },
 
@@ -681,10 +679,10 @@ var UI = {
     controlbarHandleMouseDown: function(e) {
         if ((e.type == "mousedown") && (e.button != 0)) return;
 
-        var ptr = getPointerEvent(e);
+        const ptr = getPointerEvent(e);
 
-        var handle = document.getElementById("noVNC_control_bar_handle");
-        var bounds = handle.getBoundingClientRect();
+        const handle = document.getElementById("noVNC_control_bar_handle");
+        const bounds = handle.getBoundingClientRect();
 
         // Touch events have implicit capture
         if (e.type === "mousedown") {
@@ -721,7 +719,7 @@ var UI = {
     // Initial page load read/initialization of settings
     initSetting: function(name, defVal) {
         // Check Query string followed by cookie
-        var val = WebUtil.getConfigVar(name);
+        let val = WebUtil.getConfigVar(name);
         if (val === null) {
             val = WebUtil.readSetting(name, defVal);
         }
@@ -735,14 +733,14 @@ var UI = {
     updateSetting: function(name) {
 
         // Update the settings control
-        var value = UI.getSetting(name);
+        let value = UI.getSetting(name);
 
-        var ctrl = document.getElementById('noVNC_setting_' + name);
+        const ctrl = document.getElementById('noVNC_setting_' + name);
         if (ctrl.type === 'checkbox') {
             ctrl.checked = value;
 
         } else if (typeof ctrl.options !== 'undefined') {
-            for (var i = 0; i < ctrl.options.length; i += 1) {
+            for (let i = 0; i < ctrl.options.length; i += 1) {
                 if (ctrl.options[i].value === value) {
                     ctrl.selectedIndex = i;
                     break;
@@ -760,7 +758,8 @@ var UI = {
 
     // Save control setting to cookie
     saveSetting: function(name) {
-        var val, ctrl = document.getElementById('noVNC_setting_' + name);
+        const ctrl = document.getElementById('noVNC_setting_' + name);
+        let val;
         if (ctrl.type === 'checkbox') {
             val = ctrl.checked;
         } else if (typeof ctrl.options !== 'undefined') {
@@ -775,8 +774,8 @@ var UI = {
 
     // Read form control compatible setting from cookie
     getSetting: function(name) {
-        var ctrl = document.getElementById('noVNC_setting_' + name);
-        var val = WebUtil.readSetting(name);
+        const ctrl = document.getElementById('noVNC_setting_' + name);
+        let val = WebUtil.readSetting(name);
         if (typeof val !== 'undefined' && val !== null && ctrl.type === 'checkbox') {
             if (val.toString().toLowerCase() in {'0':1, 'no':1, 'false':1}) {
                 val = false;
@@ -791,13 +790,13 @@ var UI = {
     // previous-sibling-selectors in CSS which are needed when we want to
     // disable the labels that belong to disabled input elements.
     disableSetting: function(name) {
-        var ctrl = document.getElementById('noVNC_setting_' + name);
+        const ctrl = document.getElementById('noVNC_setting_' + name);
         ctrl.disabled = true;
         ctrl.label.classList.add('noVNC_disabled');
     },
 
     enableSetting: function(name) {
-        var ctrl = document.getElementById('noVNC_setting_' + name);
+        const ctrl = document.getElementById('noVNC_setting_' + name);
         ctrl.disabled = false;
         ctrl.label.classList.remove('noVNC_disabled');
     },
@@ -950,7 +949,7 @@ var UI = {
     },
 
     clipboardSend: function() {
-        var text = document.getElementById('noVNC_clipboard_text').value;
+        const text = document.getElementById('noVNC_clipboard_text').value;
         Log.Debug(">> UI.clipboardSend: " + text.substr(0,40) + "...");
         UI.rfb.clipboardPasteFrom(text);
         Log.Debug("<< UI.clipboardSend");
@@ -979,9 +978,9 @@ var UI = {
             return;
         }
 
-        var host = UI.getSetting('host');
-        var port = UI.getSetting('port');
-        var path = UI.getSetting('path');
+        const host = UI.getSetting('host');
+        const port = UI.getSetting('port');
+        const path = UI.getSetting('path');
 
         if (typeof password === 'undefined') {
             password = WebUtil.getConfigVar('password');
@@ -1005,7 +1004,7 @@ var UI = {
 
         UI.updateVisualState('connecting');
 
-        var url;
+        let url;
 
         url = UI.getSetting('encrypt') ? 'wss' : 'ws';
 
@@ -1089,7 +1088,7 @@ var UI = {
     },
 
     disconnectFinished: function (e) {
-        let wasConnected = UI.connected;
+        const wasConnected = UI.connected;
 
         // This variable is ideally set when disconnection starts, but
         // when the disconnection isn't clean or if it is initiated by
@@ -1110,7 +1109,7 @@ var UI = {
         } else if (UI.getSetting('reconnect', false) === true && !UI.inhibit_reconnect) {
             UI.updateVisualState('reconnecting');
 
-            var delay = parseInt(UI.getSetting('reconnect_delay'));
+            const delay = parseInt(UI.getSetting('reconnect_delay'));
             UI.reconnect_callback = setTimeout(UI.reconnect, delay);
             return;
         } else {
@@ -1159,8 +1158,8 @@ var UI = {
         // Prevent actually submitting the form
         e.preventDefault();
 
-        var inputElem = document.getElementById('noVNC_password_input');
-        var password = inputElem.value;
+        const inputElem = document.getElementById('noVNC_password_input');
+        const password = inputElem.value;
         // Clear the input after reading the password
         inputElem.value = "";
         UI.rfb.sendCredentials({ password: password });
@@ -1241,8 +1240,8 @@ var UI = {
     updateViewClip: function() {
         if (!UI.rfb) return;
 
-        var cur_clip = UI.rfb.clipViewport;
-        var new_clip = UI.getSetting('view_clip');
+        const cur_clip = UI.rfb.clipViewport;
+        let new_clip = UI.getSetting('view_clip');
 
         if (isTouchDevice) {
             // Touch devices usually have shit scrollbars
@@ -1260,7 +1259,7 @@ var UI = {
 
     // Handle special cases where viewport clipping is forced on/off or locked
     enableDisableViewClip: function() {
-        var resizeSetting = UI.getSetting('resize');
+        const resizeSetting = UI.getSetting('resize');
         // Disable clipping if we are scaling, connected or on touch
         if (resizeSetting === 'scale' ||
             isTouchDevice) {
@@ -1279,7 +1278,7 @@ var UI = {
     toggleViewDrag: function() {
         if (!UI.rfb) return;
 
-        var drag = UI.rfb.dragViewport;
+        const drag = UI.rfb.dragViewport;
         UI.setViewDrag(!drag);
      },
 
@@ -1295,7 +1294,7 @@ var UI = {
     updateViewDrag: function() {
         if (!UI.connected) return;
 
-        var viewDragButton = document.getElementById('noVNC_view_drag_button');
+        const viewDragButton = document.getElementById('noVNC_view_drag_button');
 
         if (!UI.rfb.clipViewport && UI.rfb.dragViewport) {
             // We are no longer clipping the viewport. Make sure
@@ -1339,14 +1338,14 @@ var UI = {
     showVirtualKeyboard: function() {
         if (!isTouchDevice) return;
 
-        var input = document.getElementById('noVNC_keyboardinput');
+        const input = document.getElementById('noVNC_keyboardinput');
 
         if (document.activeElement == input) return;
 
         input.focus();
 
         try {
-            var l = input.value.length;
+            const l = input.value.length;
             // Move the caret to the end
             input.setSelectionRange(l, l);
         } catch (err) {
@@ -1357,7 +1356,7 @@ var UI = {
     hideVirtualKeyboard: function() {
         if (!isTouchDevice) return;
 
-        var input = document.getElementById('noVNC_keyboardinput');
+        const input = document.getElementById('noVNC_keyboardinput');
 
         if (document.activeElement != input) return;
 
@@ -1390,7 +1389,7 @@ var UI = {
     },
 
     keepVirtualKeyboard: function(event) {
-        var input = document.getElementById('noVNC_keyboardinput');
+        const input = document.getElementById('noVNC_keyboardinput');
 
         // Only prevent focus change if the virtual keyboard is active
         if (document.activeElement != input) {
@@ -1418,7 +1417,7 @@ var UI = {
     },
 
     keyboardinputReset: function() {
-        var kbi = document.getElementById('noVNC_keyboardinput');
+        const kbi = document.getElementById('noVNC_keyboardinput');
         kbi.value = new Array(UI.defaultKeyboardinputLen).join("_");
         UI.lastKeyboardinput = kbi.value;
     },
@@ -1437,14 +1436,14 @@ var UI = {
 
         if (!UI.rfb) return;
 
-        var newValue = event.target.value;
+        const newValue = event.target.value;
 
         if (!UI.lastKeyboardinput) {
             UI.keyboardinputReset();
         }
-        var oldValue = UI.lastKeyboardinput;
+        const oldValue = UI.lastKeyboardinput;
 
-        var newLen;
+        let newLen;
         try {
             // Try to check caret position since whitespace at the end
             // will not be considered by value.length in some browsers
@@ -1453,20 +1452,14 @@ var UI = {
             // selectionStart is undefined in Google Chrome
             newLen = newValue.length;
         }
-        var oldLen = oldValue.length;
+        const oldLen = oldValue.length;
 
-        var backspaces;
-        var inputs = newLen - oldLen;
-        if (inputs < 0) {
-            backspaces = -inputs;
-        } else {
-            backspaces = 0;
-        }
+        let inputs = newLen - oldLen;
+        let backspaces = inputs < 0 ? -inputs : 0;
 
         // Compare the old string with the new to account for
         // text-corrections or other input that modify existing text
-        var i;
-        for (i = 0; i < Math.min(oldLen, newLen); i++) {
+        for (let i = 0; i < Math.min(oldLen, newLen); i++) {
             if (newValue.charAt(i) != oldValue.charAt(i)) {
                 inputs = newLen - i;
                 backspaces = oldLen - i;
@@ -1475,10 +1468,10 @@ var UI = {
         }
 
         // Send the key events
-        for (i = 0; i < backspaces; i++) {
+        for (let i = 0; i < backspaces; i++) {
             UI.rfb.sendKey(KeyTable.XK_BackSpace, "Backspace");
         }
-        for (i = newLen - inputs; i < newLen; i++) {
+        for (let i = newLen - inputs; i < newLen; i++) {
             UI.rfb.sendKey(keysyms.lookup(newValue.charCodeAt(i)));
         }
 
@@ -1541,7 +1534,7 @@ var UI = {
     },
 
     toggleCtrl: function() {
-        var btn = document.getElementById('noVNC_toggle_ctrl_button');
+        const btn = document.getElementById('noVNC_toggle_ctrl_button');
         if (btn.classList.contains("noVNC_selected")) {
             UI.rfb.sendKey(KeyTable.XK_Control_L, "ControlLeft", false);
             btn.classList.remove("noVNC_selected");
@@ -1552,7 +1545,7 @@ var UI = {
     },
 
     toggleAlt: function() {
-        var btn = document.getElementById('noVNC_toggle_alt_button');
+        const btn = document.getElementById('noVNC_toggle_alt_button');
         if (btn.classList.contains("noVNC_selected")) {
             UI.rfb.sendKey(KeyTable.XK_Alt_L, "AltLeft", false);
             btn.classList.remove("noVNC_selected");
@@ -1573,14 +1566,14 @@ var UI = {
  * ------v------*/
 
     setMouseButton: function(num) {
-        var view_only = UI.rfb.viewOnly;
+        const view_only = UI.rfb.viewOnly;
         if (UI.rfb && !view_only) {
             UI.rfb.touchButton = num;
         }
 
-        var blist = [0, 1,2,4];
-        for (var b = 0; b < blist.length; b++) {
-            var button = document.getElementById('noVNC_mouse_button' +
+        const blist = [0, 1,2,4];
+        for (let b = 0; b < blist.length; b++) {
+            const button = document.getElementById('noVNC_mouse_button' +
                                                  blist[b]);
             if (blist[b] === num && !view_only) {
                 button.classList.remove("noVNC_hidden");
@@ -1621,7 +1614,7 @@ var UI = {
 
     bell: function(e) {
         if (WebUtil.getConfigVar('bell', 'on') === 'on') {
-            var promise = document.getElementById('noVNC_bell').play();
+            const promise = document.getElementById('noVNC_bell').play();
             // The standards disagree on the return value here
             if (promise) {
                 promise.catch(function(e) {
@@ -1639,7 +1632,7 @@ var UI = {
 
     //Helper to add options to dropdown.
     addOption: function(selectbox, text, value) {
-        var optn = document.createElement("OPTION");
+        const optn = document.createElement("OPTION");
         optn.text = text;
         optn.value = value;
         selectbox.options.add(optn);
@@ -1652,7 +1645,7 @@ var UI = {
 };
 
 // Set up translations
-var LINGUAS = ["de", "el", "es", "nl", "pl", "sv", "tr", "zh_CN", "zh_TW"];
+const LINGUAS = ["de", "el", "es", "nl", "pl", "sv", "tr", "zh_CN", "zh_TW"];
 l10n.setup(LINGUAS);
 if (l10n.language !== "en" && l10n.dictionary === undefined) {
     WebUtil.fetchJSON('app/locale/' + l10n.language + '.json', function (translations) {
diff --git a/app/webutil.js b/app/webutil.js
index 73d24af..18816d1 100644
--- a/app/webutil.js
+++ b/app/webutil.js
@@ -15,7 +15,7 @@ export function init_logging (level) {
     if (typeof level !== "undefined") {
         main_init_logging(level);
     } else {
-        var param = document.location.href.match(/logging=([A-Za-z0-9._-]*)/);
+        const param = document.location.href.match(/logging=([A-Za-z0-9._-]*)/);
         main_init_logging(param || undefined);
     }
 }
@@ -23,37 +23,41 @@ export function init_logging (level) {
 // Read a query string variable
 export function getQueryVar (name, defVal) {
     "use strict";
-    var re = new RegExp('.*[?&]' + name + '=([^&#]*)'),
+    const re = new RegExp('.*[?&]' + name + '=([^&#]*)'),
         match = document.location.href.match(re);
     if (typeof defVal === 'undefined') { defVal = null; }
+    
     if (match) {
         return decodeURIComponent(match[1]);
-    } else {
-        return defVal;
     }
+
+    return defVal;
 }
 
 // Read a hash fragment variable
 export function getHashVar (name, defVal) {
     "use strict";
-    var re = new RegExp('.*[&#]' + name + '=([^&]*)'),
+    const re = new RegExp('.*[&#]' + name + '=([^&]*)'),
         match = document.location.hash.match(re);
     if (typeof defVal === 'undefined') { defVal = null; }
+
     if (match) {
         return decodeURIComponent(match[1]);
-    } else {
-        return defVal;
     }
+
+    return defVal;
 }
 
 // Read a variable from the fragment or the query string
 // Fragment takes precedence
 export function getConfigVar (name, defVal) {
     "use strict";
-    var val = getHashVar(name);
+    const val = getHashVar(name);
+
     if (val === null) {
-        val = getQueryVar(name, defVal);
+        return getQueryVar(name, defVal);
     }
+
     return val;
 }
 
@@ -64,7 +68,7 @@ export function getConfigVar (name, defVal) {
 // No days means only for this browser session
 export function createCookie (name, value, days) {
     "use strict";
-    var date, expires;
+    let date, expires;
     if (days) {
         date = new Date();
         date.setTime(date.getTime() + (days * 24 * 60 * 60 * 1000));
@@ -73,7 +77,7 @@ export function createCookie (name, value, days) {
         expires = "";
     }
 
-    var secure;
+    let secure;
     if (document.location.protocol === "https:") {
         secure = "; secure";
     } else {
@@ -84,14 +88,19 @@ export function createCookie (name, value, days) {
 
 export function readCookie (name, defaultValue) {
     "use strict";
-    var nameEQ = name + "=",
-        ca = document.cookie.split(';');
+    const nameEQ = name + "=";
+    const ca = document.cookie.split(';');
 
-    for (var i = 0; i < ca.length; i += 1) {
-        var c = ca[i];
-        while (c.charAt(0) === ' ') { c = c.substring(1, c.length); }
-        if (c.indexOf(nameEQ) === 0) { return c.substring(nameEQ.length, c.length); }
+    for (let i = 0; i < ca.length; i += 1) {
+        let c = ca[i];
+        while (c.charAt(0) === ' ') {
+            c = c.substring(1, c.length);
+        }
+        if (c.indexOf(nameEQ) === 0) {
+            return c.substring(nameEQ.length, c.length);
+        }
     }
+
     return (typeof defaultValue !== 'undefined') ? defaultValue : null;
 }
 
@@ -104,11 +113,11 @@ export function eraseCookie (name) {
  * Setting handling.
  */
 
-var settings = {};
+let settings = {};
 
 export function initSettings (callback /*, ...callbackArgs */) {
     "use strict";
-    var callbackArgs = Array.prototype.slice.call(arguments, 1);
+    const callbackArgs = Array.prototype.slice.call(arguments, 1);
     if (window.chrome && window.chrome.storage) {
         window.chrome.storage.sync.get(function (cfg) {
             settings = cfg;
@@ -143,7 +152,7 @@ export function writeSetting (name, value) {
 
 export function readSetting (name, defaultValue) {
     "use strict";
-    var value;
+    let value;
     if ((name in settings) || (window.chrome && window.chrome.storage)) {
         value = settings[name];
     } else {
@@ -153,11 +162,12 @@ export function readSetting (name, defaultValue) {
     if (typeof value === "undefined") {
         value = null;
     }
+
     if (value === null && typeof defaultValue !== "undefined") {
         return defaultValue;
-    } else {
-        return value;
     }
+
+    return value;
 }
 
 export function eraseSetting (name) {
@@ -180,11 +190,11 @@ export function injectParamIfMissing (path, param, value) {
     // (assume that we wanted an extra if we pass one in)
     path = "/" + path;
 
-    var elem = document.createElement('a');
+    const elem = document.createElement('a');
     elem.href = path;
 
-    var param_eq = encodeURIComponent(param) + "=";
-    var query;
+    const param_eq = encodeURIComponent(param) + "=";
+    let query;
     if (elem.search) {
         query = elem.search.slice(1).split('&');
     } else {
@@ -200,9 +210,9 @@ export function injectParamIfMissing (path, param, value) {
     // in the elem.pathname string. Handle that case gracefully.
     if (elem.pathname.charAt(0) == "/") {
         return elem.pathname.slice(1) + elem.search + elem.hash;
-    } else {
-        return elem.pathname + elem.search + elem.hash;
     }
+
+    return elem.pathname + elem.search + elem.hash;
 }
 
 // sadly, we can't use the Fetch API until we decide to drop
@@ -211,16 +221,16 @@ export function injectParamIfMissing (path, param, value) {
 // will receive either an event or an error on failure.
 export function fetchJSON(path, resolve, reject) {
     // NB: IE11 doesn't support JSON as a responseType
-    var req = new XMLHttpRequest();
+    const req = new XMLHttpRequest();
     req.open('GET', path);
 
     req.onload = function () {
         if (req.status === 200) {
+            let resObj;
             try {
-                var resObj = JSON.parse(req.responseText);
+                resObj = JSON.parse(req.responseText);
             } catch (err) {
                 reject(err);
-                return;
             }
             resolve(resObj);
         } else {
diff --git a/core/base64.js b/core/base64.js
index 48e28c3..d57d986 100644
--- a/core/base64.js
+++ b/core/base64.js
@@ -13,33 +13,30 @@ export default {
 
     encode: function (data) {
         "use strict";
-        var result = '';
-        var toBase64Table = this.toBase64Table;
-        var length = data.length;
-        var lengthpad = (length % 3);
+        let result = '';
+        const length = data.length;
+        const lengthpad = (length % 3);
         // Convert every three bytes to 4 ascii characters.
 
-        for (var i = 0; i < (length - 2); i += 3) {
-            result += toBase64Table[data[i] >> 2];
-            result += toBase64Table[((data[i] & 0x03) << 4) + (data[i + 1] >> 4)];
-            result += toBase64Table[((data[i + 1] & 0x0f) << 2) + (data[i + 2] >> 6)];
-            result += toBase64Table[data[i + 2] & 0x3f];
+        for (let i = 0; i < (length - 2); i += 3) {
+            result += this.toBase64Table[data[i] >> 2];
+            result += this.toBase64Table[((data[i] & 0x03) << 4) + (data[i + 1] >> 4)];
+            result += this.toBase64Table[((data[i + 1] & 0x0f) << 2) + (data[i + 2] >> 6)];
+            result += this.toBase64Table[data[i + 2] & 0x3f];
         }
 
         // Convert the remaining 1 or 2 bytes, pad out to 4 characters.
-        var j = 0;
+        const j = length - lengthpad;
         if (lengthpad === 2) {
-            j = length - lengthpad;
-            result += toBase64Table[data[j] >> 2];
-            result += toBase64Table[((data[j] & 0x03) << 4) + (data[j + 1] >> 4)];
-            result += toBase64Table[(data[j + 1] & 0x0f) << 2];
-            result += toBase64Table[64];
+            result += this.toBase64Table[data[j] >> 2];
+            result += this.toBase64Table[((data[j] & 0x03) << 4) + (data[j + 1] >> 4)];
+            result += this.toBase64Table[(data[j + 1] & 0x0f) << 2];
+            result += this.toBase64Table[64];
         } else if (lengthpad === 1) {
-            j = length - lengthpad;
-            result += toBase64Table[data[j] >> 2];
-            result += toBase64Table[(data[j] & 0x03) << 4];
-            result += toBase64Table[64];
-            result += toBase64Table[64];
+            result += this.toBase64Table[data[j] >> 2];
+            result += this.toBase64Table[(data[j] & 0x03) << 4];
+            result += this.toBase64Table[64];
+            result += this.toBase64Table[64];
         }
 
         return result;
@@ -60,23 +57,21 @@ export default {
     decode: function (data, offset) {
         "use strict";
         offset = typeof(offset) !== 'undefined' ? offset : 0;
-        var toBinaryTable = this.toBinaryTable;
-        var base64Pad = this.base64Pad;
-        var result, result_length;
-        var leftbits = 0; // number of bits decoded, but yet to be appended
-        var leftdata = 0; // bits decoded, but yet to be appended
-        var data_length = data.indexOf('=') - offset;
 
+        let data_length = data.indexOf('=') - offset;
         if (data_length < 0) { data_length = data.length - offset; }
 
         /* Every four characters is 3 resulting numbers */
-        result_length = (data_length >> 2) * 3 + Math.floor((data_length % 4) / 1.5);
-        result = new Array(result_length);
+        const result_length = (data_length >> 2) * 3 + Math.floor((data_length % 4) / 1.5);
+        const result = new Array(result_length);
 
         // Convert one by one.
-        for (var idx = 0, i = offset; i < data.length; i++) {
-            var c = toBinaryTable[data.charCodeAt(i) & 0x7f];
-            var padding = (data.charAt(i) === base64Pad);
+
+        let leftbits = 0; // number of bits decoded, but yet to be appended
+        let leftdata = 0; // bits decoded, but yet to be appended
+        for (let idx = 0, i = offset; i < data.length; i++) {
+            const c = this.toBinaryTable[data.charCodeAt(i) & 0x7f];
+            const padding = (data.charAt(i) === this.base64Pad);
             // Skip illegal characters and whitespace
             if (c === -1) {
                 Log.Error("Illegal character code " + data.charCodeAt(i) + " at position " + i);
@@ -100,7 +95,7 @@ export default {
 
         // If there are any bits left, the base64 string was corrupted
         if (leftbits) {
-            var err = new Error('Corrupted base64 string');
+            const err = new Error('Corrupted base64 string');
             err.name = 'Base64-Error';
             throw err;
         }
diff --git a/core/des.js b/core/des.js
index 821929b..5adc7ae 100644
--- a/core/des.js
+++ b/core/des.js
@@ -79,80 +79,76 @@ export default function DES(passwd) {
     "use strict";
 
     // Tables, permutations, S-boxes, etc.
-    var PC2 = [13,16,10,23, 0, 4, 2,27,14, 5,20, 9,22,18,11, 3,
-               25, 7,15, 6,26,19,12, 1,40,51,30,36,46,54,29,39,
-               50,44,32,47,43,48,38,55,33,52,45,41,49,35,28,31 ],
+    const PC2 = [13,16,10,23, 0, 4, 2,27,14, 5,20, 9,22,18,11, 3,
+                 25, 7,15, 6,26,19,12, 1,40,51,30,36,46,54,29,39,
+                 50,44,32,47,43,48,38,55,33,52,45,41,49,35,28,31 ],
         totrot = [ 1, 2, 4, 6, 8,10,12,14,15,17,19,21,23,25,27,28],
-        z = 0x0, a,b,c,d,e,f, SP1,SP2,SP3,SP4,SP5,SP6,SP7,SP8,
+        z = 0x0,
         keys = [];
+    let a,b,c,d,e,f;
 
     a=1<<16; b=1<<24; c=a|b; d=1<<2; e=1<<10; f=d|e;
-    SP1 = [c|e,z|z,a|z,c|f,c|d,a|f,z|d,a|z,z|e,c|e,c|f,z|e,b|f,c|d,b|z,z|d,
-           z|f,b|e,b|e,a|e,a|e,c|z,c|z,b|f,a|d,b|d,b|d,a|d,z|z,z|f,a|f,b|z,
-           a|z,c|f,z|d,c|z,c|e,b|z,b|z,z|e,c|d,a|z,a|e,b|d,z|e,z|d,b|f,a|f,
-           c|f,a|d,c|z,b|f,b|d,z|f,a|f,c|e,z|f,b|e,b|e,z|z,a|d,a|e,z|z,c|d];
+    const SP1 = [c|e,z|z,a|z,c|f,c|d,a|f,z|d,a|z,z|e,c|e,c|f,z|e,b|f,c|d,b|z,z|d,
+                 z|f,b|e,b|e,a|e,a|e,c|z,c|z,b|f,a|d,b|d,b|d,a|d,z|z,z|f,a|f,b|z,
+                 a|z,c|f,z|d,c|z,c|e,b|z,b|z,z|e,c|d,a|z,a|e,b|d,z|e,z|d,b|f,a|f,
+                 c|f,a|d,c|z,b|f,b|d,z|f,a|f,c|e,z|f,b|e,b|e,z|z,a|d,a|e,z|z,c|d];
     a=1<<20; b=1<<31; c=a|b; d=1<<5; e=1<<15; f=d|e;
-    SP2 = [c|f,b|e,z|e,a|f,a|z,z|d,c|d,b|f,b|d,c|f,c|e,b|z,b|e,a|z,z|d,c|d,
-           a|e,a|d,b|f,z|z,b|z,z|e,a|f,c|z,a|d,b|d,z|z,a|e,z|f,c|e,c|z,z|f,
-           z|z,a|f,c|d,a|z,b|f,c|z,c|e,z|e,c|z,b|e,z|d,c|f,a|f,z|d,z|e,b|z,
-           z|f,c|e,a|z,b|d,a|d,b|f,b|d,a|d,a|e,z|z,b|e,z|f,b|z,c|d,c|f,a|e];
+    const SP2 = [c|f,b|e,z|e,a|f,a|z,z|d,c|d,b|f,b|d,c|f,c|e,b|z,b|e,a|z,z|d,c|d,
+                 a|e,a|d,b|f,z|z,b|z,z|e,a|f,c|z,a|d,b|d,z|z,a|e,z|f,c|e,c|z,z|f,
+                 z|z,a|f,c|d,a|z,b|f,c|z,c|e,z|e,c|z,b|e,z|d,c|f,a|f,z|d,z|e,b|z,
+                 z|f,c|e,a|z,b|d,a|d,b|f,b|d,a|d,a|e,z|z,b|e,z|f,b|z,c|d,c|f,a|e];
     a=1<<17; b=1<<27; c=a|b; d=1<<3; e=1<<9; f=d|e;
-    SP3 = [z|f,c|e,z|z,c|d,b|e,z|z,a|f,b|e,a|d,b|d,b|d,a|z,c|f,a|d,c|z,z|f,
-           b|z,z|d,c|e,z|e,a|e,c|z,c|d,a|f,b|f,a|e,a|z,b|f,z|d,c|f,z|e,b|z,
-           c|e,b|z,a|d,z|f,a|z,c|e,b|e,z|z,z|e,a|d,c|f,b|e,b|d,z|e,z|z,c|d,
-           b|f,a|z,b|z,c|f,z|d,a|f,a|e,b|d,c|z,b|f,z|f,c|z,a|f,z|d,c|d,a|e];
+    const SP3 = [z|f,c|e,z|z,c|d,b|e,z|z,a|f,b|e,a|d,b|d,b|d,a|z,c|f,a|d,c|z,z|f,
+                 b|z,z|d,c|e,z|e,a|e,c|z,c|d,a|f,b|f,a|e,a|z,b|f,z|d,c|f,z|e,b|z,
+                 c|e,b|z,a|d,z|f,a|z,c|e,b|e,z|z,z|e,a|d,c|f,b|e,b|d,z|e,z|z,c|d,
+                 b|f,a|z,b|z,c|f,z|d,a|f,a|e,b|d,c|z,b|f,z|f,c|z,a|f,z|d,c|d,a|e];
     a=1<<13; b=1<<23; c=a|b; d=1<<0; e=1<<7; f=d|e;
-    SP4 = [c|d,a|f,a|f,z|e,c|e,b|f,b|d,a|d,z|z,c|z,c|z,c|f,z|f,z|z,b|e,b|d,
-           z|d,a|z,b|z,c|d,z|e,b|z,a|d,a|e,b|f,z|d,a|e,b|e,a|z,c|e,c|f,z|f,
-           b|e,b|d,c|z,c|f,z|f,z|z,z|z,c|z,a|e,b|e,b|f,z|d,c|d,a|f,a|f,z|e,
-           c|f,z|f,z|d,a|z,b|d,a|d,c|e,b|f,a|d,a|e,b|z,c|d,z|e,b|z,a|z,c|e];
+    const SP4 = [c|d,a|f,a|f,z|e,c|e,b|f,b|d,a|d,z|z,c|z,c|z,c|f,z|f,z|z,b|e,b|d,
+                 z|d,a|z,b|z,c|d,z|e,b|z,a|d,a|e,b|f,z|d,a|e,b|e,a|z,c|e,c|f,z|f,
+                 b|e,b|d,c|z,c|f,z|f,z|z,z|z,c|z,a|e,b|e,b|f,z|d,c|d,a|f,a|f,z|e,
+                 c|f,z|f,z|d,a|z,b|d,a|d,c|e,b|f,a|d,a|e,b|z,c|d,z|e,b|z,a|z,c|e];
     a=1<<25; b=1<<30; c=a|b; d=1<<8; e=1<<19; f=d|e;
-    SP5 = [z|d,a|f,a|e,c|d,z|e,z|d,b|z,a|e,b|f,z|e,a|d,b|f,c|d,c|e,z|f,b|z,
-           a|z,b|e,b|e,z|z,b|d,c|f,c|f,a|d,c|e,b|d,z|z,c|z,a|f,a|z,c|z,z|f,
-           z|e,c|d,z|d,a|z,b|z,a|e,c|d,b|f,a|d,b|z,c|e,a|f,b|f,z|d,a|z,c|e,
-           c|f,z|f,c|z,c|f,a|e,z|z,b|e,c|z,z|f,a|d,b|d,z|e,z|z,b|e,a|f,b|d];
+    const SP5 = [z|d,a|f,a|e,c|d,z|e,z|d,b|z,a|e,b|f,z|e,a|d,b|f,c|d,c|e,z|f,b|z,
+                 a|z,b|e,b|e,z|z,b|d,c|f,c|f,a|d,c|e,b|d,z|z,c|z,a|f,a|z,c|z,z|f,
+                 z|e,c|d,z|d,a|z,b|z,a|e,c|d,b|f,a|d,b|z,c|e,a|f,b|f,z|d,a|z,c|e,
+                 c|f,z|f,c|z,c|f,a|e,z|z,b|e,c|z,z|f,a|d,b|d,z|e,z|z,b|e,a|f,b|d];
     a=1<<22; b=1<<29; c=a|b; d=1<<4; e=1<<14; f=d|e;
-    SP6 = [b|d,c|z,z|e,c|f,c|z,z|d,c|f,a|z,b|e,a|f,a|z,b|d,a|d,b|e,b|z,z|f,
-           z|z,a|d,b|f,z|e,a|e,b|f,z|d,c|d,c|d,z|z,a|f,c|e,z|f,a|e,c|e,b|z,
-           b|e,z|d,c|d,a|e,c|f,a|z,z|f,b|d,a|z,b|e,b|z,z|f,b|d,c|f,a|e,c|z,
-           a|f,c|e,z|z,c|d,z|d,z|e,c|z,a|f,z|e,a|d,b|f,z|z,c|e,b|z,a|d,b|f];
+    const SP6 = [b|d,c|z,z|e,c|f,c|z,z|d,c|f,a|z,b|e,a|f,a|z,b|d,a|d,b|e,b|z,z|f,
+                 z|z,a|d,b|f,z|e,a|e,b|f,z|d,c|d,c|d,z|z,a|f,c|e,z|f,a|e,c|e,b|z,
+                 b|e,z|d,c|d,a|e,c|f,a|z,z|f,b|d,a|z,b|e,b|z,z|f,b|d,c|f,a|e,c|z,
+                 a|f,c|e,z|z,c|d,z|d,z|e,c|z,a|f,z|e,a|d,b|f,z|z,c|e,b|z,a|d,b|f];
     a=1<<21; b=1<<26; c=a|b; d=1<<1; e=1<<11; f=d|e;
-    SP7 = [a|z,c|d,b|f,z|z,z|e,b|f,a|f,c|e,c|f,a|z,z|z,b|d,z|d,b|z,c|d,z|f,
-           b|e,a|f,a|d,b|e,b|d,c|z,c|e,a|d,c|z,z|e,z|f,c|f,a|e,z|d,b|z,a|e,
-           b|z,a|e,a|z,b|f,b|f,c|d,c|d,z|d,a|d,b|z,b|e,a|z,c|e,z|f,a|f,c|e,
-           z|f,b|d,c|f,c|z,a|e,z|z,z|d,c|f,z|z,a|f,c|z,z|e,b|d,b|e,z|e,a|d];
+    const SP7 = [a|z,c|d,b|f,z|z,z|e,b|f,a|f,c|e,c|f,a|z,z|z,b|d,z|d,b|z,c|d,z|f,
+                 b|e,a|f,a|d,b|e,b|d,c|z,c|e,a|d,c|z,z|e,z|f,c|f,a|e,z|d,b|z,a|e,
+                 b|z,a|e,a|z,b|f,b|f,c|d,c|d,z|d,a|d,b|z,b|e,a|z,c|e,z|f,a|f,c|e,
+                 z|f,b|d,c|f,c|z,a|e,z|z,z|d,c|f,z|z,a|f,c|z,z|e,b|d,b|e,z|e,a|d];
     a=1<<18; b=1<<28; c=a|b; d=1<<6; e=1<<12; f=d|e;
-    SP8 = [b|f,z|e,a|z,c|f,b|z,b|f,z|d,b|z,a|d,c|z,c|f,a|e,c|e,a|f,z|e,z|d,
-           c|z,b|d,b|e,z|f,a|e,a|d,c|d,c|e,z|f,z|z,z|z,c|d,b|d,b|e,a|f,a|z,
-           a|f,a|z,c|e,z|e,z|d,c|d,z|e,a|f,b|e,z|d,b|d,c|z,c|d,b|z,a|z,b|f,
-           z|z,c|f,a|d,b|d,c|z,b|e,b|f,z|z,c|f,a|e,a|e,z|f,z|f,a|d,b|z,c|e];
+    const SP8 = [b|f,z|e,a|z,c|f,b|z,b|f,z|d,b|z,a|d,c|z,c|f,a|e,c|e,a|f,z|e,z|d,
+                 c|z,b|d,b|e,z|f,a|e,a|d,c|d,c|e,z|f,z|z,z|z,c|d,b|d,b|e,a|f,a|z,
+                 a|f,a|z,c|e,z|e,z|d,c|d,z|e,a|f,b|e,z|d,b|d,c|z,c|d,b|z,a|z,b|f,
+                 z|z,c|f,a|d,b|d,c|z,b|e,b|f,z|z,c|f,a|e,a|e,z|f,z|f,a|d,b|z,c|e];
 
     // Set the key.
     function setKeys(keyBlock) {
-        var i, j, l, m, n, o, pc1m = [], pcr = [], kn = [],
-            raw0, raw1, rawi, KnLi;
+        const pc1m = [], pcr = [], kn = [];
 
-        for (j = 0, l = 56; j < 56; ++j, l -= 8) {
+        for (let j = 0, l = 56; j < 56; ++j, l -= 8) {
             l += l < -5 ? 65 : l < -3 ? 31 : l < -1 ? 63 : l === 27 ? 35 : 0; // PC1
-            m = l & 0x7;
+            const m = l & 0x7;
             pc1m[j] = ((keyBlock[l >>> 3] & (1<<m)) !== 0) ? 1: 0;
         }
 
-        for (i = 0; i < 16; ++i) {
-            m = i << 1;
-            n = m + 1;
+        for (let i = 0; i < 16; ++i) {
+            const m = i << 1;
+            const n = m + 1;
             kn[m] = kn[n] = 0;
-            for (o = 28; o < 59; o += 28) {
-                for (j = o - 28; j < o; ++j) {
-                    l = j + totrot[i];
-                    if (l < o) {
-                        pcr[j] = pc1m[l];
-                    } else {
-                        pcr[j] = pc1m[l - 28];
-                    }
+            for (let o = 28; o < 59; o += 28) {
+                for (let j = o - 28; j < o; ++j) {
+                    const l = j + totrot[i];
+                    pcr[j] = l < o ? pc1m[l] : pc1m[l - 28];
                 }
             }
-            for (j = 0; j < 24; ++j) {
+            for (let j = 0; j < 24; ++j) {
                 if (pcr[PC2[j]] !== 0) {
                     kn[m] |= 1 << (23 - j);
                 }
@@ -163,9 +159,9 @@ export default function DES(passwd) {
         }
 
         // cookey
-        for (i = 0, rawi = 0, KnLi = 0; i < 16; ++i) {
-            raw0 = kn[rawi++];
-            raw1 = kn[rawi++];
+        for (let i = 0, rawi = 0, KnLi = 0; i < 16; ++i) {
+            const raw0 = kn[rawi++];
+            const raw1 = kn[rawi++];
             keys[KnLi] = (raw0 & 0x00fc0000) << 6;
             keys[KnLi] |= (raw0 & 0x00000fc0) << 10;
             keys[KnLi] |= (raw1 & 0x00fc0000) >>> 10;
@@ -181,8 +177,8 @@ export default function DES(passwd) {
 
     // Encrypt 8 bytes of text
     function enc8(text) {
-        var i = 0, b = text.slice(), fval, keysi = 0,
-            l, r, x; // left, right, accumulator
+        const b = text.slice();
+        let i = 0, l, r, x; // left, right, accumulator
 
         // Squash 8 bytes to 2 ints
         l = b[i++]<<24 | b[i++]<<16 | b[i++]<<8 | b[i++];
@@ -206,10 +202,10 @@ export default function DES(passwd) {
         r ^= x;
         l = (l << 1) | ((l >>> 31) & 1);
 
-        for (i = 0; i < 8; ++i) {
+        for (let i = 0, keysi = 0; i < 8; ++i) {
             x = (r << 28) | (r >>> 4);
             x ^= keys[keysi++];
-            fval =  SP7[x & 0x3f];
+            let fval =  SP7[x & 0x3f];
             fval |= SP5[(x >>> 8) & 0x3f];
             fval |= SP3[(x >>> 16) & 0x3f];
             fval |= SP1[(x >>> 24) & 0x3f];
diff --git a/core/display.js b/core/display.js
index 7fed184..eb7eec2 100644
--- a/core/display.js
+++ b/core/display.js
@@ -70,7 +70,7 @@ export default function Display(target) {
     Log.Debug("<< Display.constructor");
 }
 
-var SUPPORTS_IMAGEDATA_CONSTRUCTOR = false;
+let SUPPORTS_IMAGEDATA_CONSTRUCTOR = false;
 try {
     new ImageData(new Uint8ClampedArray(4), 1, 1);
     SUPPORTS_IMAGEDATA_CONSTRUCTOR = true;
@@ -92,7 +92,7 @@ Display.prototype = {
     set clipViewport(viewport) {
         this._clipViewport = viewport;
         // May need to readjust the viewport dimensions
-        var vp = this._viewportLoc;
+        const vp = this._viewportLoc;
         this.viewportChangeSize(vp.w, vp.h);
         this.viewportChangePos(0, 0);
     },
@@ -113,7 +113,7 @@ Display.prototype = {
     // ===== PUBLIC METHODS =====
 
     viewportChangePos: function (deltaX, deltaY) {
-        var vp = this._viewportLoc;
+        const vp = this._viewportLoc;
         deltaX = Math.floor(deltaX);
         deltaY = Math.floor(deltaY);
 
@@ -122,8 +122,8 @@ Display.prototype = {
             deltaY = -vp.h;
         }
 
-        var vx2 = vp.x + vp.w - 1;
-        var vy2 = vp.y + vp.h - 1;
+        const vx2 = vp.x + vp.w - 1;
+        const vy2 = vp.y + vp.h - 1;
 
         // Position change
 
@@ -172,12 +172,12 @@ Display.prototype = {
             height = this._fb_height;
         }
 
-        var vp = this._viewportLoc;
+        const vp = this._viewportLoc;
         if (vp.w !== width || vp.h !== height) {
             vp.w = width;
             vp.h = height;
 
-            var canvas = this._target;
+            const canvas = this._target;
             canvas.width = width;
             canvas.height = height;
 
@@ -206,11 +206,11 @@ Display.prototype = {
         this._fb_width = width;
         this._fb_height = height;
 
-        var canvas = this._backbuffer;
+        const canvas = this._backbuffer;
         if (canvas.width !== width || canvas.height !== height) {
 
             // We have to save the canvas data since changing the size will clear it
-            var saveImg = null;
+            let saveImg = null;
             if (canvas.width > 0 && canvas.height > 0) {
                 saveImg = this._drawCtx.getImageData(0, 0, canvas.width, canvas.height);
             }
@@ -229,7 +229,7 @@ Display.prototype = {
 
         // Readjust the viewport as it may be incorrectly sized
         // and positioned
-        var vp = this._viewportLoc;
+        const vp = this._viewportLoc;
         this.viewportChangeSize(vp.w, vp.h);
         this.viewportChangePos(0, 0);
     },
@@ -258,15 +258,13 @@ Display.prototype = {
                 'type': 'flip'
             });
         } else {
-            var x, y, vx, vy, w, h;
+            let x = this._damageBounds.left;
+            let y = this._damageBounds.top;
+            let w = this._damageBounds.right - x;
+            let h = this._damageBounds.bottom - y;
 
-            x = this._damageBounds.left;
-            y = this._damageBounds.top;
-            w = this._damageBounds.right - x;
-            h = this._damageBounds.bottom - y;
-
-            vx = x - this._viewportLoc.x;
-            vy = y - this._viewportLoc.y;
+            let vx = x - this._viewportLoc.x;
+            let vy = y - this._viewportLoc.y;
 
             if (vx < 0) {
                 w += vx;
@@ -372,7 +370,7 @@ Display.prototype = {
     },
 
     imageRect: function(x, y, mime, arr) {
-        var img = new Image();
+        const img = new Image();
         img.src = "data: " + mime + ";base64," + Base64.encode(arr);
         this._renderQ_push({
             'type': 'img',
@@ -392,12 +390,12 @@ Display.prototype = {
             this._tile = this._drawCtx.createImageData(width, height);
         }
 
-        var red = color[2];
-        var green = color[1];
-        var blue = color[0];
+        const red = color[2];
+        const green = color[1];
+        const blue = color[0];
 
-        var data = this._tile.data;
-        for (var i = 0; i < width * height * 4; i += 4) {
+        const data = this._tile.data;
+        for (let i = 0; i < width * height * 4; i += 4) {
             data[i] = red;
             data[i + 1] = green;
             data[i + 2] = blue;
@@ -407,17 +405,17 @@ Display.prototype = {
 
     // update sub-rectangle of the current tile
     subTile: function (x, y, w, h, color) {
-        var red = color[2];
-        var green = color[1];
-        var blue = color[0];
-        var xend = x + w;
-        var yend = y + h;
-
-        var data = this._tile.data;
-        var width = this._tile.width;
-        for (var j = y; j < yend; j++) {
-            for (var i = x; i < xend; i++) {
-                var p = (i + (j * width)) * 4;
+        const red = color[2];
+        const green = color[1];
+        const blue = color[0];
+        const xend = x + w;
+        const yend = y + h;
+
+        const data = this._tile.data;
+        const width = this._tile.width;
+        for (let j = y; j < yend; j++) {
+            for (let i = x; i < xend; i++) {
+                const p = (i + (j * width)) * 4;
                 data[p] = red;
                 data[p + 1] = green;
                 data[p + 2] = blue;
@@ -438,7 +436,7 @@ Display.prototype = {
             // NB(directxman12): it's technically more performant here to use preallocated arrays,
             // but it's a lot of extra work for not a lot of payoff -- if we're using the render queue,
             // this probably isn't getting called *nearly* as much
-            var new_arr = new Uint8Array(width * height * 4);
+            const new_arr = new Uint8Array(width * height * 4);
             new_arr.set(new Uint8Array(arr.buffer, 0, new_arr.length));
             this._renderQ_push({
                 'type': 'blit',
@@ -458,7 +456,7 @@ Display.prototype = {
             // NB(directxman12): it's technically more performant here to use preallocated arrays,
             // but it's a lot of extra work for not a lot of payoff -- if we're using the render queue,
             // this probably isn't getting called *nearly* as much
-            var new_arr = new Uint8Array(width * height * 3);
+            const new_arr = new Uint8Array(width * height * 3);
             new_arr.set(new Uint8Array(arr.buffer, 0, new_arr.length));
             this._renderQ_push({
                 'type': 'blitRgb',
@@ -478,7 +476,7 @@ Display.prototype = {
             // NB(directxman12): it's technically more performant here to use preallocated arrays,
             // but it's a lot of extra work for not a lot of payoff -- if we're using the render queue,
             // this probably isn't getting called *nearly* as much
-            var new_arr = new Uint8Array(width * height * 4);
+            const new_arr = new Uint8Array(width * height * 4);
             new_arr.set(new Uint8Array(arr.buffer, 0, new_arr.length));
             this._renderQ_push({
                 'type': 'blitRgbx',
@@ -511,11 +509,11 @@ Display.prototype = {
     },
 
     autoscale: function (containerWidth, containerHeight) {
-        var vp = this._viewportLoc;
-        var targetAspectRatio = containerWidth / containerHeight;
-        var fbAspectRatio = vp.w / vp.h;
+        const vp = this._viewportLoc;
+        const targetAspectRatio = containerWidth / containerHeight;
+        const fbAspectRatio = vp.w / vp.h;
 
-        var scaleRatio;
+        let scaleRatio;
         if (fbAspectRatio >= targetAspectRatio) {
             scaleRatio = containerWidth / vp.w;
         } else {
@@ -529,14 +527,14 @@ Display.prototype = {
 
     _rescale: function (factor) {
         this._scale = factor;
-        var vp = this._viewportLoc;
+        const vp = this._viewportLoc;
 
         // NB(directxman12): If you set the width directly, or set the
         //                   style width to a number, the canvas is cleared.
         //                   However, if you set the style width to a string
         //                   ('NNNpx'), the canvas is scaled without clearing.
-        var width = Math.round(factor * vp.w) + 'px';
-        var height = Math.round(factor * vp.h) + 'px';
+        const width = Math.round(factor * vp.w) + 'px';
+        const height = Math.round(factor * vp.h) + 'px';
 
         if ((this._target.style.width !== width) ||
             (this._target.style.height !== height)) {
@@ -546,7 +544,7 @@ Display.prototype = {
     },
 
     _setFillColor: function (color) {
-        var newStyle = 'rgb(' + color[2] + ',' + color[1] + ',' + color[0] + ')';
+        const newStyle = 'rgb(' + color[2] + ',' + color[1] + ',' + color[0] + ')';
         if (newStyle !== this._prevDrawStyle) {
             this._drawCtx.fillStyle = newStyle;
             this._prevDrawStyle = newStyle;
@@ -554,9 +552,9 @@ Display.prototype = {
     },
 
     _rgbImageData: function (x, y, width, height, arr, offset) {
-        var img = this._drawCtx.createImageData(width, height);
-        var data = img.data;
-        for (var i = 0, j = offset; i < width * height * 4; i += 4, j += 3) {
+        const img = this._drawCtx.createImageData(width, height);
+        const data = img.data;
+        for (let i = 0, j = offset; i < width * height * 4; i += 4, j += 3) {
             data[i]     = arr[j];
             data[i + 1] = arr[j + 1];
             data[i + 2] = arr[j + 2];
@@ -567,9 +565,9 @@ Display.prototype = {
     },
 
     _bgrxImageData: function (x, y, width, height, arr, offset) {
-        var img = this._drawCtx.createImageData(width, height);
-        var data = img.data;
-        for (var i = 0, j = offset; i < width * height * 4; i += 4, j += 4) {
+        const img = this._drawCtx.createImageData(width, height);
+        const data = img.data;
+        for (let i = 0, j = offset; i < width * height * 4; i += 4, j += 4) {
             data[i]     = arr[j + 2];
             data[i + 1] = arr[j + 1];
             data[i + 2] = arr[j];
@@ -581,7 +579,7 @@ Display.prototype = {
 
     _rgbxImageData: function (x, y, width, height, arr, offset) {
         // NB(directxman12): arr must be an Type Array view
-        var img;
+        let img;
         if (SUPPORTS_IMAGEDATA_CONSTRUCTOR) {
             img = new ImageData(new Uint8ClampedArray(arr.buffer, arr.byteOffset, width * height * 4), width, height);
         } else {
@@ -609,9 +607,9 @@ Display.prototype = {
     },
 
     _scan_renderQ: function () {
-        var ready = true;
+        let ready = true;
         while (ready && this._renderQ.length > 0) {
-            var a = this._renderQ[0];
+            const a = this._renderQ[0];
             switch (a.type) {
                 case 'flip':
                     this.flip(true);
@@ -663,12 +661,11 @@ Display.changeCursor = function (target, pixels, mask, hotx, hoty, w, h) {
         return;
     }
 
-    var cur = []
-    var y, x;
-    for (y = 0; y < h; y++) {
-        for (x = 0; x < w; x++) {
-            var idx = y * Math.ceil(w / 8) + Math.floor(x / 8);
-            var alpha = (mask[idx] << (x % 8)) & 0x80 ? 255 : 0;
+    const cur = []
+    for (let y = 0; y < h; y++) {
+        for (let x = 0; x < w; x++) {
+            let idx = y * Math.ceil(w / 8) + Math.floor(x / 8);
+            const alpha = (mask[idx] << (x % 8)) & 0x80 ? 255 : 0;
             idx = ((w * y) + x) * 4;
             cur.push(pixels[idx + 2]); // red
             cur.push(pixels[idx + 1]); // green
@@ -677,13 +674,13 @@ Display.changeCursor = function (target, pixels, mask, hotx, hoty, w, h) {
         }
     }
 
-    var canvas = document.createElement('canvas');
-    var ctx = canvas.getContext('2d');
+    const canvas = document.createElement('canvas');
+    const ctx = canvas.getContext('2d');
 
     canvas.width = w;
     canvas.height = h;
 
-    var img;
+    let img;
     if (SUPPORTS_IMAGEDATA_CONSTRUCTOR) {
         img = new ImageData(new Uint8ClampedArray(cur), w, h);
     } else {
@@ -693,6 +690,6 @@ Display.changeCursor = function (target, pixels, mask, hotx, hoty, w, h) {
     ctx.clearRect(0, 0, w, h);
     ctx.putImageData(img, 0, 0);
 
-    var url = canvas.toDataURL();
+    const url = canvas.toDataURL();
     target.style.cursor = 'url(' + url + ')' + hotx + ' ' + hoty + ', default';
 };
diff --git a/core/encodings.js b/core/encodings.js
index c383063..5a70e66 100644
--- a/core/encodings.js
+++ b/core/encodings.js
@@ -6,7 +6,7 @@
  * See README.md for usage and integration instructions.
  */
 
-export var encodings = {
+export const encodings = {
     encodingRaw: 0,
     encodingCopyRect: 1,
     encodingRRE: 2,
diff --git a/core/input/domkeytable.js b/core/input/domkeytable.js
index 7103bba..a0cc1f5 100644
--- a/core/input/domkeytable.js
+++ b/core/input/domkeytable.js
@@ -13,7 +13,7 @@ import KeyTable from "./keysym.js";
  * See https://www.w3.org/TR/uievents-key/ for possible values.
  */
 
-var DOMKeyTable = {};
+const DOMKeyTable = {};
 
 function addStandard(key, standard)
 {
diff --git a/core/input/keyboard.js b/core/input/keyboard.js
index 4ba36e3..d64298a 100644
--- a/core/input/keyboard.js
+++ b/core/input/keyboard.js
@@ -57,7 +57,7 @@ Keyboard.prototype = {
     },
 
     _getKeyCode: function (e) {
-        var code = KeyboardUtil.getKeycode(e);
+        const code = KeyboardUtil.getKeycode(e);
         if (code !== 'Unidentified') {
             return code;
         }
@@ -80,10 +80,8 @@ Keyboard.prototype = {
                 return e.keyIdentifier;
             }
 
-            var codepoint = parseInt(e.keyIdentifier.substr(2), 16);
-            var char = String.fromCharCode(codepoint);
-            // Some implementations fail to uppercase the symbols
-            char = char.toUpperCase();
+            const codepoint = parseInt(e.keyIdentifier.substr(2), 16);
+            const char = String.fromCharCode(codepoint).toUpperCase();
 
             return 'Platform' + char.charCodeAt();
         }
@@ -92,8 +90,8 @@ Keyboard.prototype = {
     },
 
     _handleKeyDown: function (e) {
-        var code = this._getKeyCode(e);
-        var keysym = KeyboardUtil.getKeysym(e);
+        const code = this._getKeyCode(e);
+        let keysym = KeyboardUtil.getKeysym(e);
 
         // Windows doesn't have a proper AltGr, but handles it using
         // fake Ctrl+Alt. However the remote end might not be Windows,
@@ -211,8 +209,8 @@ Keyboard.prototype = {
             return;
         }
 
-        var code = this._getKeyCode(e);
-        var keysym = KeyboardUtil.getKeysym(e);
+        let code = this._getKeyCode(e);
+        const keysym = KeyboardUtil.getKeysym(e);
 
         // The key we were waiting for?
         if ((code !== 'Unidentified') && (code != this._pendingKey)) {
@@ -235,9 +233,9 @@ Keyboard.prototype = {
             return;
         }
 
-        var code, keysym;
+        let keysym;
 
-        code = this._pendingKey;
+        const code = this._pendingKey;
         this._pendingKey = null;
 
         // We have no way of knowing the proper keysym with the
@@ -248,7 +246,7 @@ Keyboard.prototype = {
             keysym = e.keyCode;
         } else if ((e.keyCode >= 0x41) && (e.keyCode <= 0x5a)) {
             // Character (A-Z)
-            var char = String.fromCharCode(e.keyCode);
+            let char = String.fromCharCode(e.keyCode);
             // A feeble attempt at the correct case
             if (e.shiftKey)
                 char = char.toUpperCase();
@@ -266,7 +264,7 @@ Keyboard.prototype = {
     _handleKeyUp: function (e) {
         stopEvent(e);
 
-        var code = this._getKeyCode(e);
+        const code = this._getKeyCode(e);
 
         // We can't get a release in the middle of an AltGr sequence, so
         // abort that detection
@@ -294,7 +292,7 @@ Keyboard.prototype = {
 
     _allKeysUp: function () {
         Log.Debug(">> Keyboard.allKeysUp");
-        for (var code in this._keyDownList) {
+        for (let code in this._keyDownList) {
             this._sendKeyEvent(this._keyDownList[code], code, false);
         }
         Log.Debug("<< Keyboard.allKeysUp");
@@ -306,14 +304,14 @@ Keyboard.prototype = {
             return;
         }
 
-        let target = this._target;
-        let downList = this._keyDownList;
+        const target = this._target;
+        const downList = this._keyDownList;
         ['AltLeft', 'AltRight'].forEach(function (code) {
             if (!(code in downList)) {
                 return;
             }
 
-            let event = new KeyboardEvent('keyup',
+            const event = new KeyboardEvent('keyup',
                                           { key: downList[code],
                                             code: code });
             target.dispatchEvent(event);
@@ -324,11 +322,10 @@ Keyboard.prototype = {
 
     grab: function () {
         //Log.Debug(">> Keyboard.grab");
-        var c = this._target;
 
-        c.addEventListener('keydown', this._eventHandlers.keydown);
-        c.addEventListener('keyup', this._eventHandlers.keyup);
-        c.addEventListener('keypress', this._eventHandlers.keypress);
+        this._target.addEventListener('keydown', this._eventHandlers.keydown);
+        this._target.addEventListener('keyup', this._eventHandlers.keyup);
+        this._target.addEventListener('keypress', this._eventHandlers.keypress);
 
         // Release (key up) if window loses focus
         window.addEventListener('blur', this._eventHandlers.blur);
@@ -337,7 +334,7 @@ Keyboard.prototype = {
         // best we can for releases (still doesn't prevent the menu
         // from popping up though as we can't call preventDefault())
         if (browser.isWindows() && browser.isFirefox()) {
-            let handler = this._eventHandlers.checkalt;
+            const handler = this._eventHandlers.checkalt;
             ['mousedown', 'mouseup', 'mousemove', 'wheel',
              'touchstart', 'touchend', 'touchmove',
              'keydown', 'keyup'].forEach(function (type) {
@@ -352,10 +349,9 @@ Keyboard.prototype = {
 
     ungrab: function () {
         //Log.Debug(">> Keyboard.ungrab");
-        var c = this._target;
 
         if (browser.isWindows() && browser.isFirefox()) {
-            let handler = this._eventHandlers.checkalt;
+            const handler = this._eventHandlers.checkalt;
             ['mousedown', 'mouseup', 'mousemove', 'wheel',
              'touchstart', 'touchend', 'touchmove',
              'keydown', 'keyup'].forEach(function (type) {
@@ -363,9 +359,9 @@ Keyboard.prototype = {
             });
         }
 
-        c.removeEventListener('keydown', this._eventHandlers.keydown);
-        c.removeEventListener('keyup', this._eventHandlers.keyup);
-        c.removeEventListener('keypress', this._eventHandlers.keypress);
+        this._target.removeEventListener('keydown', this._eventHandlers.keydown);
+        this._target.removeEventListener('keyup', this._eventHandlers.keyup);
+        this._target.removeEventListener('keypress', this._eventHandlers.keypress);
         window.removeEventListener('blur', this._eventHandlers.blur);
 
         // Release (key up) all keys that are in a down state
diff --git a/core/input/keysymdef.js b/core/input/keysymdef.js
index 95922b3..5969b5d 100644
--- a/core/input/keysymdef.js
+++ b/core/input/keysymdef.js
@@ -7,7 +7,7 @@
 
 /* Functions at the bottom */
 
-var codepoints = {
+const codepoints = {
     0x0100: 0x03c0, // XK_Amacron
     0x0101: 0x03e0, // XK_amacron
     0x0102: 0x01c3, // XK_Abreve
@@ -677,7 +677,7 @@ export default {
         }
 
         // Lookup table (fairly random)
-        var keysym = codepoints[u];
+        const keysym = codepoints[u];
         if (keysym !== undefined) {
             return keysym;
         }
diff --git a/core/input/mouse.js b/core/input/mouse.js
index df9b7e1..17c8cd5 100644
--- a/core/input/mouse.js
+++ b/core/input/mouse.js
@@ -9,9 +9,9 @@ import * as Log from '../util/logging.js';
 import { isTouchDevice } from '../util/browser.js';
 import { setCapture, stopEvent, getPointerEvent } from '../util/events.js';
 
-var WHEEL_STEP = 10; // Delta threshold for a mouse wheel step
-var WHEEL_STEP_TIMEOUT = 50; // ms
-var WHEEL_LINE_HEIGHT = 19;
+const WHEEL_STEP = 10; // Delta threshold for a mouse wheel step
+const WHEEL_STEP_TIMEOUT = 50; // ms
+const WHEEL_LINE_HEIGHT = 19;
 
 export default function Mouse(target) {
     this._target = target || document;
@@ -52,9 +52,9 @@ Mouse.prototype = {
 
     _handleMouseButton: function (e, down) {
         this._updateMousePosition(e);
-        var pos = this._pos;
+        let pos = this._pos;
 
-        var bmask;
+        let bmask;
         if (e.touches || e.changedTouches) {
             // Touch device
 
@@ -70,13 +70,13 @@ Mouse.prototype = {
                     // force the position of the latter touch to the position of
                     // the first.
 
-                    var xs = this._lastTouchPos.x - pos.x;
-                    var ys = this._lastTouchPos.y - pos.y;
-                    var d = Math.sqrt((xs * xs) + (ys * ys));
+                    const xs = this._lastTouchPos.x - pos.x;
+                    const ys = this._lastTouchPos.y - pos.y;
+                    const d = Math.sqrt((xs * xs) + (ys * ys));
 
                     // The goal is to trigger on a certain physical width, the
                     // devicePixelRatio brings us a bit closer but is not optimal.
-                    var threshold = 20 * (window.devicePixelRatio || 1);
+                    const threshold = 20 * (window.devicePixelRatio || 1);
                     if (d < threshold) {
                         pos = this._lastTouchPos;
                     }
@@ -156,8 +156,8 @@ Mouse.prototype = {
 
         this._updateMousePosition(e);
 
-        var dX = e.deltaX;
-        var dY = e.deltaY;
+        let dX = e.deltaX;
+        let dY = e.deltaY;
 
         // Pixel units unless it's non-zero.
         // Note that if deltamode is line or page won't matter since we aren't
@@ -215,8 +215,9 @@ Mouse.prototype = {
     // Update coordinates relative to target
     _updateMousePosition: function(e) {
         e = getPointerEvent(e);
-        var bounds = this._target.getBoundingClientRect();
-        var x, y;
+        const bounds = this._target.getBoundingClientRect();
+        let x;
+        let y;
         // Clip to target bounds
         if (e.clientX < bounds.left) {
             x = 0;
@@ -238,7 +239,7 @@ Mouse.prototype = {
     // ===== PUBLIC METHODS =====
 
     grab: function () {
-        var c = this._target;
+        const c = this._target;
 
         if (isTouchDevice) {
             c.addEventListener('touchstart', this._eventHandlers.mousedown);
@@ -259,7 +260,7 @@ Mouse.prototype = {
     },
 
     ungrab: function () {
-        var c = this._target;
+        const c = this._target;
 
         this._resetWheelStepTimers();
 
diff --git a/core/input/util.js b/core/input/util.js
index 1201e1d..57e9ce4 100644
--- a/core/input/util.js
+++ b/core/input/util.js
@@ -24,7 +24,7 @@ export function getKeycode(evt){
     // in the 'keyCode' field for non-printable characters. However
     // Webkit sets it to the same as charCode in 'keypress' events.
     if ((evt.type !== 'keypress') && (evt.keyCode in vkeys)) {
-        var code = vkeys[evt.keyCode];
+        let code = vkeys[evt.keyCode];
 
         // macOS has messed up this code for some reason
         if (browser.isMac() && (code === 'ContextMenu')) {
@@ -110,7 +110,7 @@ export function getKey(evt) {
     }
 
     // Try to deduce it based on the physical key
-    var code = getKeycode(evt);
+    const code = getKeycode(evt);
     if (code in fixedkeys) {
         return fixedkeys[code];
     }
@@ -126,7 +126,7 @@ export function getKey(evt) {
 
 // Get the most reliable keysym value we can get from a key event
 export function getKeysym(evt){
-    var key = getKey(evt);
+    const key = getKey(evt);
 
     if (key === 'Unidentified') {
         return null;
@@ -134,7 +134,7 @@ export function getKeysym(evt){
 
     // First look up special keys
     if (key in DOMKeyTable) {
-        var location = evt.location;
+        let location = evt.location;
 
         // Safari screws up location for the right cmd key
         if ((key === 'Meta') && (location === 0)) {
@@ -150,14 +150,12 @@ export function getKeysym(evt){
 
     // Now we need to look at the Unicode symbol instead
 
-    var codepoint;
-
     // Special key? (FIXME: Should have been caught earlier)
     if (key.length !== 1) {
         return null;
     }
 
-    codepoint = key.charCodeAt();
+    const codepoint = key.charCodeAt();
     if (codepoint) {
         return keysyms.lookup(codepoint);
     }
diff --git a/core/rfb.js b/core/rfb.js
index 652fbeb..d336d6c 100644
--- a/core/rfb.js
+++ b/core/rfb.js
@@ -26,7 +26,7 @@ import { encodings, encodingName } from "./encodings.js";
 import "./util/polyfill.js";
 
 // How many seconds to wait for a disconnect to finish
-var DISCONNECT_TIMEOUT = 3;
+const DISCONNECT_TIMEOUT = 3;
 
 export default function RFB(target, url, options) {
     if (!target) {
@@ -105,7 +105,7 @@ export default function RFB(target, url, options) {
         background: null,
         zlibs: []               // TIGHT zlib streams
     };
-    for (var i = 0; i < 4; i++) {
+    for (let i = 0; i < 4; i++) {
         this._FBU.zlibs[i] = new Inflator();
     }
 
@@ -207,7 +207,7 @@ export default function RFB(target, url, options) {
     }.bind(this));
     this._sock.on('close', function (e) {
         Log.Debug("WebSocket on-close event");
-        var msg = "";
+        let msg = "";
         if (e.code) {
             msg = "(code: " + e.code;
             if (e.reason) {
@@ -358,7 +358,7 @@ RFB.prototype = {
             return;
         }
 
-        var scancode = XtScancode[code];
+        const scancode = XtScancode[code];
 
         if (this._qemuExtKeyEventSupported && scancode) {
             // 0 is NoSymbol
@@ -445,11 +445,11 @@ RFB.prototype = {
     },
 
     _print_stats: function () {
-        var stats = this._encStats;
+        const stats = this._encStats;
 
         Log.Info("Encoding stats for this connection:");
         Object.keys(stats).forEach(function (key) {
-            var s = stats[key];
+            const s = stats[key];
             if (s[0] + s[1] > 0) {
                 Log.Info("    " + encodingName(key) + ": " + s[0] + " rects");
             }
@@ -457,7 +457,7 @@ RFB.prototype = {
 
         Log.Info("Encoding stats since page load:");
         Object.keys(stats).forEach(function (key) {
-            var s = stats[key];
+            const s = stats[key];
             Log.Info("    " + encodingName(key) + ": " + s[1] + " rects");
         });
     },
@@ -497,8 +497,8 @@ RFB.prototype = {
     // Update state of clipping in Display object, and make sure the
     // configured viewport matches the current screen size
     _updateClip: function () {
-        var cur_clip = this._display.clipViewport;
-        var new_clip = this._clipViewport;
+        const cur_clip = this._display.clipViewport;
+        let new_clip = this._clipViewport;
 
         if (this._scaleViewport) {
             // Disable viewport clipping if we are scaling
@@ -512,7 +512,7 @@ RFB.prototype = {
         if (new_clip) {
             // When clipping is enabled, the screen is limited to
             // the size of the container.
-            let size = this._screenSize();
+            const size = this._screenSize();
             this._display.viewportChangeSize(size.w, size.h);
             this._fixScrollbars();
         }
@@ -522,7 +522,7 @@ RFB.prototype = {
         if (!this._scaleViewport) {
             this._display.scale = 1.0;
         } else {
-            let size = this._screenSize();
+            const size = this._screenSize();
             this._display.autoscale(size.w, size.h);
         }
         this._fixScrollbars();
@@ -539,7 +539,7 @@ RFB.prototype = {
             return;
         }
 
-        let size = this._screenSize();
+        const size = this._screenSize();
         RFB.messages.setDesktopSize(this._sock, size.w, size.h,
                                     this._screen_id, this._screen_flags);
 
@@ -557,7 +557,7 @@ RFB.prototype = {
         // This is a hack because Chrome screws up the calculation
         // for when scrollbars are needed. So to fix it we temporarily
         // toggle them off and on.
-        var orig = this._screen.style.overflow;
+        const orig = this._screen.style.overflow;
         this._screen.style.overflow = 'hidden';
         // Force Chrome to recalculate the layout by asking for
         // an element's dimensions
@@ -573,7 +573,7 @@ RFB.prototype = {
      *   disconnected - permanent state
      */
     _updateConnectionState: function (state) {
-        var oldstate = this._rfb_connection_state;
+        const oldstate = this._rfb_connection_state;
 
         if (state === oldstate) {
             Log.Debug("Already in state '" + state + "', ignoring");
@@ -629,7 +629,7 @@ RFB.prototype = {
 
         this._rfb_connection_state = state;
 
-        var smsg = "New state '" + state + "', was '" + oldstate + "'.";
+        const smsg = "New state '" + state + "', was '" + oldstate + "'.";
         Log.Debug(smsg);
 
         if (this._disconnTimer && state !== 'disconnecting') {
@@ -647,8 +647,7 @@ RFB.prototype = {
                 break;
 
             case 'connected':
-                var event = new CustomEvent("connect", { detail: {} });
-                this.dispatchEvent(event);
+                this.dispatchEvent(new CustomEvent("connect", { detail: {} }));
                 break;
 
             case 'disconnecting':
@@ -661,10 +660,9 @@ RFB.prototype = {
                 break;
 
             case 'disconnected':
-                event = new CustomEvent(
+                this.dispatchEvent(new CustomEvent(
                     "disconnect", { detail:
-                                    { clean: this._rfb_clean_disconnect } });
-                this.dispatchEvent(event);
+                                    { clean: this._rfb_clean_disconnect } }));
                 break;
         }
     },
@@ -700,9 +698,8 @@ RFB.prototype = {
 
     _setCapability: function (cap, val) {
         this._capabilities[cap] = val;
-        var event = new CustomEvent("capabilities",
-                                    { detail: { capabilities: this._capabilities } });
-        this.dispatchEvent(event);
+        this.dispatchEvent(new CustomEvent("capabilities",
+            { detail: { capabilities: this._capabilities } }));
     },
 
     _handle_message: function () {
@@ -780,12 +777,12 @@ RFB.prototype = {
 
     _handleMouseMove: function (x, y) {
         if (this._viewportDragging) {
-            var deltaX = this._viewportDragPos.x - x;
-            var deltaY = this._viewportDragPos.y - y;
+            const deltaX = this._viewportDragPos.x - x;
+            const deltaY = this._viewportDragPos.y - y;
 
             // The goal is to trigger on a certain physical width, the
             // devicePixelRatio brings us a bit closer but is not optimal.
-            var dragThreshold = 10 * (window.devicePixelRatio || 1);
+            const dragThreshold = 10 * (window.devicePixelRatio || 1);
 
             if (this._viewportHasMoved || (Math.abs(deltaX) > dragThreshold ||
                                            Math.abs(deltaY) > dragThreshold)) {
@@ -812,9 +809,9 @@ RFB.prototype = {
             return this._fail("Received incomplete protocol version.");
         }
 
-        var sversion = this._sock.rQshiftStr(12).substr(4, 7);
+        const sversion = this._sock.rQshiftStr(12).substr(4, 7);
         Log.Info("Server ProtocolVersion: " + sversion);
-        var is_repeater = 0;
+        let is_repeater = 0;
         switch (sversion) {
             case "000.000":  // UltraVNC repeater
                 is_repeater = 1;
@@ -838,7 +835,7 @@ RFB.prototype = {
         }
 
         if (is_repeater) {
-            var repeaterID = "ID:" + this._repeaterID;
+            let repeaterID = "ID:" + this._repeaterID;
             while (repeaterID.length < 250) {
                 repeaterID += "\0";
             }
@@ -850,7 +847,7 @@ RFB.prototype = {
             this._rfb_version = this._rfb_max_version;
         }
 
-        var cversion = "00" + parseInt(this._rfb_version, 10) +
+        const cversion = "00" + parseInt(this._rfb_version, 10) +
                        ".00" + ((this._rfb_version * 10) % 10);
         this._sock.send_string("RFB " + cversion + "\n");
         Log.Debug('Sent ProtocolVersion: ' + cversion);
@@ -862,7 +859,7 @@ RFB.prototype = {
         // Polyfill since IE and PhantomJS doesn't have
         // TypedArray.includes()
         function includes(item, array) {
-            for (var i = 0; i < array.length; i++) {
+            for (let i = 0; i < array.length; i++) {
                 if (array[i] === item) {
                     return true;
                 }
@@ -872,14 +869,14 @@ RFB.prototype = {
 
         if (this._rfb_version >= 3.7) {
             // Server sends supported list, client decides
-            var num_types = this._sock.rQshift8();
+            const num_types = this._sock.rQshift8();
             if (this._sock.rQwait("security type", num_types, 1)) { return false; }
 
             if (num_types === 0) {
                 return this._handle_security_failure("no security types");
             }
 
-            var types = this._sock.rQshiftBytes(num_types);
+            const types = this._sock.rQshiftBytes(num_types);
             Log.Debug("Server security types: " + types);
 
             // Look for each auth in preferred order
@@ -934,7 +931,7 @@ RFB.prototype = {
         if (this._sock.rQwait("reason length", 4)) {
             return false;
         }
-        let strlen = this._sock.rQshift32();
+        const strlen = this._sock.rQshift32();
         let reason = "";
 
         if (strlen > 0) {
@@ -943,20 +940,16 @@ RFB.prototype = {
         }
 
         if (reason !== "") {
-
-            let event = new CustomEvent(
+            this.dispatchEvent(new CustomEvent(
                 "securityfailure",
-                { detail: { status: security_result_status, reason: reason } });
-            this.dispatchEvent(event);
+                { detail: { status: security_result_status, reason: reason } }));
 
             return this._fail("Security negotiation failed" + context +
                               " (reason: " + reason + ")");
         } else {
-
-            let event = new CustomEvent(
+            this.dispatchEvent(new CustomEvent(
                 "securityfailure",
-                { detail: { status: security_result_status } });
-            this.dispatchEvent(event);
+                { detail: { status: security_result_status } }));
 
             return this._fail("Security negotiation failed" + context);
         }
@@ -967,13 +960,13 @@ RFB.prototype = {
         if (!this._rfb_credentials.username ||
             !this._rfb_credentials.password ||
             !this._rfb_credentials.target) {
-            var event = new CustomEvent("credentialsrequired",
-                                        { detail: { types: ["username", "password", "target"] } });
-            this.dispatchEvent(event);
+            this.dispatchEvent(new CustomEvent(
+                "credentialsrequired",
+                { detail: { types: ["username", "password", "target"] } }));
             return false;
         }
 
-        var xvp_auth_str = String.fromCharCode(this._rfb_credentials.username.length) +
+        const xvp_auth_str = String.fromCharCode(this._rfb_credentials.username.length) +
                            String.fromCharCode(this._rfb_credentials.target.length) +
                            this._rfb_credentials.username +
                            this._rfb_credentials.target;
@@ -986,30 +979,30 @@ RFB.prototype = {
         if (this._sock.rQwait("auth challenge", 16)) { return false; }
 
         if (!this._rfb_credentials.password) {
-            var event = new CustomEvent("credentialsrequired",
-                                        { detail: { types: ["password"] } });
-            this.dispatchEvent(event);
+            this.dispatchEvent(new CustomEvent(
+                "credentialsrequired",
+                { detail: { types: ["password"] } }));
             return false;
         }
 
         // TODO(directxman12): make genDES not require an Array
-        var challenge = Array.prototype.slice.call(this._sock.rQshiftBytes(16));
-        var response = RFB.genDES(this._rfb_credentials.password, challenge);
+        const challenge = Array.prototype.slice.call(this._sock.rQshiftBytes(16));
+        const response = RFB.genDES(this._rfb_credentials.password, challenge);
         this._sock.send(response);
         this._rfb_init_state = "SecurityResult";
         return true;
     },
 
     _negotiate_tight_tunnels: function (numTunnels) {
-        var clientSupportedTunnelTypes = {
+        const clientSupportedTunnelTypes = {
             0: { vendor: 'TGHT', signature: 'NOTUNNEL' }
         };
-        var serverSupportedTunnelTypes = {};
+        const serverSupportedTunnelTypes = {};
         // receive tunnel capabilities
-        for (var i = 0; i < numTunnels; i++) {
-            var cap_code = this._sock.rQshift32();
-            var cap_vendor = this._sock.rQshiftStr(4);
-            var cap_signature = this._sock.rQshiftStr(8);
+        for (let i = 0; i < numTunnels; i++) {
+            const cap_code = this._sock.rQshift32();
+            const cap_vendor = this._sock.rQshiftStr(4);
+            const cap_signature = this._sock.rQshiftStr(8);
             serverSupportedTunnelTypes[cap_code] = { vendor: cap_vendor, signature: cap_signature };
         }
 
@@ -1031,7 +1024,7 @@ RFB.prototype = {
     _negotiate_tight_auth: function () {
         if (!this._rfb_tightvnc) {  // first pass, do the tunnel negotiation
             if (this._sock.rQwait("num tunnels", 4)) { return false; }
-            var numTunnels = this._sock.rQshift32();
+            const numTunnels = this._sock.rQshift32();
             if (numTunnels > 0 && this._sock.rQwait("tunnel capabilities", 16 * numTunnels, 4)) { return false; }
 
             this._rfb_tightvnc = true;
@@ -1044,7 +1037,7 @@ RFB.prototype = {
 
         // second pass, do the sub-auth negotiation
         if (this._sock.rQwait("sub auth count", 4)) { return false; }
-        var subAuthCount = this._sock.rQshift32();
+        const subAuthCount = this._sock.rQshift32();
         if (subAuthCount === 0) {  // empty sub-auth list received means 'no auth' subtype selected
             this._rfb_init_state = 'SecurityResult';
             return true;
@@ -1052,20 +1045,20 @@ RFB.prototype = {
 
         if (this._sock.rQwait("sub auth capabilities", 16 * subAuthCount, 4)) { return false; }
 
-        var clientSupportedTypes = {
+        const clientSupportedTypes = {
             'STDVNOAUTH__': 1,
             'STDVVNCAUTH_': 2
         };
 
-        var serverSupportedTypes = [];
+        const serverSupportedTypes = [];
 
-        for (var i = 0; i < subAuthCount; i++) {
+        for (let i = 0; i < subAuthCount; i++) {
             this._sock.rQshift32(); // capNum
-            var capabilities = this._sock.rQshiftStr(12);
+            const capabilities = this._sock.rQshiftStr(12);
             serverSupportedTypes.push(capabilities);
         }
 
-        for (var authType in clientSupportedTypes) {
+        for (let authType in clientSupportedTypes) {
             if (serverSupportedTypes.indexOf(authType) != -1) {
                 this._sock.send([0, 0, 0, clientSupportedTypes[authType]]);
 
@@ -1117,7 +1110,7 @@ RFB.prototype = {
     _handle_security_result: function () {
         if (this._sock.rQwait('VNC auth response ', 4)) { return false; }
 
-        let status = this._sock.rQshift32();
+        const status = this._sock.rQshift32();
 
         if (status === 0) { // OK
             this._rfb_init_state = 'ClientInitialisation';
@@ -1127,9 +1120,9 @@ RFB.prototype = {
             if (this._rfb_version >= 3.8) {
                 return this._handle_security_failure("security result", status);
             } else {
-                let event = new CustomEvent("securityfailure",
-                                            { detail: { status: status } });
-                this.dispatchEvent(event);
+                this.dispatchEvent(new CustomEvent(
+                    "securityfailure",
+                    { detail: { status: status } }));
 
                 return this._fail("Security handshake failed");
             }
@@ -1140,40 +1133,40 @@ RFB.prototype = {
         if (this._sock.rQwait("server initialization", 24)) { return false; }
 
         /* Screen size */
-        var width = this._sock.rQshift16();
-        var height = this._sock.rQshift16();
+        const width = this._sock.rQshift16();
+        const height = this._sock.rQshift16();
 
         /* PIXEL_FORMAT */
-        var bpp         = this._sock.rQshift8();
-        var depth       = this._sock.rQshift8();
-        var big_endian  = this._sock.rQshift8();
-        var true_color  = this._sock.rQshift8();
-
-        var red_max     = this._sock.rQshift16();
-        var green_max   = this._sock.rQshift16();
-        var blue_max    = this._sock.rQshift16();
-        var red_shift   = this._sock.rQshift8();
-        var green_shift = this._sock.rQshift8();
-        var blue_shift  = this._sock.rQshift8();
+        const bpp         = this._sock.rQshift8();
+        const depth       = this._sock.rQshift8();
+        const big_endian  = this._sock.rQshift8();
+        const true_color  = this._sock.rQshift8();
+
+        const red_max     = this._sock.rQshift16();
+        const green_max   = this._sock.rQshift16();
+        const blue_max    = this._sock.rQshift16();
+        const red_shift   = this._sock.rQshift8();
+        const green_shift = this._sock.rQshift8();
+        const blue_shift  = this._sock.rQshift8();
         this._sock.rQskipBytes(3);  // padding
 
         // NB(directxman12): we don't want to call any callbacks or print messages until
         //                   *after* we're past the point where we could backtrack
 
         /* Connection name/title */
-        var name_length = this._sock.rQshift32();
+        const name_length = this._sock.rQshift32();
         if (this._sock.rQwait('server init name', name_length, 24)) { return false; }
         this._fb_name = decodeUTF8(this._sock.rQshiftStr(name_length));
 
         if (this._rfb_tightvnc) {
             if (this._sock.rQwait('TightVNC extended server init header', 8, 24 + name_length)) { return false; }
             // In TightVNC mode, ServerInit message is extended
-            var numServerMessages = this._sock.rQshift16();
-            var numClientMessages = this._sock.rQshift16();
-            var numEncodings = this._sock.rQshift16();
+            const numServerMessages = this._sock.rQshift16();
+            const numClientMessages = this._sock.rQshift16();
+            const numEncodings = this._sock.rQshift16();
             this._sock.rQskipBytes(2);  // padding
 
-            var totalMessagesLength = (numServerMessages + numClientMessages + numEncodings) * 16;
+            const totalMessagesLength = (numServerMessages + numClientMessages + numEncodings) * 16;
             if (this._sock.rQwait('TightVNC extended server init header', totalMessagesLength, 32 + name_length)) { return false; }
 
             // we don't actually do anything with the capability information that TIGHT sends,
@@ -1215,9 +1208,9 @@ RFB.prototype = {
         }
 
         // we're past the point where we could backtrack, so it's safe to call this
-        var event = new CustomEvent("desktopname",
-                                    { detail: { name: this._fb_name } });
-        this.dispatchEvent(event);
+        this.dispatchEvent(new CustomEvent(
+            "desktopname",
+            { detail: { name: this._fb_name } }));
 
         this._resize(width, height);
 
@@ -1247,7 +1240,7 @@ RFB.prototype = {
     },
 
     _sendEncodings: function () {
-        var encs = [];
+        const encs = [];
 
         // In preference order
         encs.push(encodings.encodingCopyRect);
@@ -1327,16 +1320,16 @@ RFB.prototype = {
 
         if (this._sock.rQwait("ServerCutText header", 7, 1)) { return false; }
         this._sock.rQskipBytes(3);  // Padding
-        var length = this._sock.rQshift32();
+        const length = this._sock.rQshift32();
         if (this._sock.rQwait("ServerCutText", length, 8)) { return false; }
 
-        var text = this._sock.rQshiftStr(length);
+        const text = this._sock.rQshiftStr(length);
 
         if (this._viewOnly) { return true; }
 
-        var event = new CustomEvent("clipboard",
-                                    { detail: { text: text } });
-        this.dispatchEvent(event);
+        this.dispatchEvent(new CustomEvent(
+            "clipboard",
+            { detail: { text: text } }));
 
         return true;
     },
@@ -1344,8 +1337,8 @@ RFB.prototype = {
     _handle_server_fence_msg: function() {
         if (this._sock.rQwait("ServerFence header", 8, 1)) { return false; }
         this._sock.rQskipBytes(3); // Padding
-        var flags = this._sock.rQshift32();
-        var length = this._sock.rQshift8();
+        let flags = this._sock.rQshift32();
+        let length = this._sock.rQshift8();
 
         if (this._sock.rQwait("ServerFence payload", length, 9)) { return false; }
 
@@ -1354,7 +1347,7 @@ RFB.prototype = {
             length = 64;
         }
 
-        var payload = this._sock.rQshiftStr(length);
+        const payload = this._sock.rQshiftStr(length);
 
         this._supportsFence = true;
 
@@ -1386,8 +1379,8 @@ RFB.prototype = {
     _handle_xvp_msg: function () {
         if (this._sock.rQwait("XVP version and message", 3, 1)) { return false; }
         this._sock.rQskip8();  // Padding
-        var xvp_ver = this._sock.rQshift8();
-        var xvp_msg = this._sock.rQshift8();
+        const xvp_ver = this._sock.rQshift8();
+        const xvp_msg = this._sock.rQshift8();
 
         switch (xvp_msg) {
             case 0:  // XVP_FAIL
@@ -1407,17 +1400,17 @@ RFB.prototype = {
     },
 
     _normal_msg: function () {
-        var msg_type;
-
+        let msg_type;
         if (this._FBU.rects > 0) {
             msg_type = 0;
         } else {
             msg_type = this._sock.rQshift8();
         }
 
+        let first, ret;
         switch (msg_type) {
             case 0:  // FramebufferUpdate
-                var ret = this._framebufferUpdate();
+                ret = this._framebufferUpdate();
                 if (ret && !this._enabledContinuousUpdates) {
                     RFB.messages.fbUpdateRequest(this._sock, true, 0, 0,
                                                  this._fb_width, this._fb_height);
@@ -1429,15 +1422,16 @@ RFB.prototype = {
 
             case 2:  // Bell
                 Log.Debug("Bell");
-                var event = new CustomEvent("bell", { detail: {} });
-                this.dispatchEvent(event);
+                this.dispatchEvent(new CustomEvent(
+                    "bell", 
+                    { detail: {} }));
                 return true;
 
             case 3:  // ServerCutText
                 return this._handle_server_cut_text();
 
             case 150: // EndOfContinuousUpdates
-                var first = !(this._supportsContinuousUpdates);
+                first = !this._supportsContinuousUpdates;
                 this._supportsContinuousUpdates = true;
                 this._enabledContinuousUpdates = false;
                 if (first) {
@@ -1472,9 +1466,6 @@ RFB.prototype = {
     },
 
     _framebufferUpdate: function () {
-        var ret = true;
-        var now;
-
         if (this._FBU.rects === 0) {
             if (this._sock.rQwait("FBU header", 3, 1)) { return false; }
             this._sock.rQskip8();  // Padding
@@ -1482,7 +1473,7 @@ RFB.prototype = {
             this._FBU.bytes = 0;
             this._timing.cur_fbu = 0;
             if (this._timing.fbu_rt_start > 0) {
-                now = (new Date()).getTime();
+                const now = (new Date()).getTime();
                 Log.Info("First FBU latency: " + (now - this._timing.fbu_rt_start));
             }
 
@@ -1503,7 +1494,7 @@ RFB.prototype = {
                 if (this._sock.rQwait("rect header", 12)) { return false; }
                 /* New FramebufferUpdate */
 
-                var hdr = this._sock.rQshiftBytes(12);
+                const hdr = this._sock.rQshiftBytes(12);
                 this._FBU.x        = (hdr[0] << 8) + hdr[1];
                 this._FBU.y        = (hdr[2] << 8) + hdr[3];
                 this._FBU.width    = (hdr[4] << 8) + hdr[5];
@@ -1520,9 +1511,9 @@ RFB.prototype = {
 
             this._timing.last_fbu = (new Date()).getTime();
 
-            ret = this._encHandlers[this._FBU.encoding]();
+            const ret = this._encHandlers[this._FBU.encoding]();
 
-            now = (new Date()).getTime();
+            const now = (new Date()).getTime();
             this._timing.cur_fbu += (now - this._timing.last_fbu);
 
             if (ret) {
@@ -1547,7 +1538,7 @@ RFB.prototype = {
                 }
 
                 if (this._timing.fbu_rt_start > 0) {
-                    var fbu_rt_diff = now - this._timing.fbu_rt_start;
+                    const fbu_rt_diff = now - this._timing.fbu_rt_start;
                     this._timing.fbu_rt_total += fbu_rt_diff;
                     this._timing.fbu_rt_cnt++;
                     Log.Info("full FBU round-trip, cur: " +
@@ -1602,8 +1593,8 @@ Object.assign(RFB.prototype, EventTargetMixin);
 // Class Methods
 RFB.messages = {
     keyEvent: function (sock, keysym, down) {
-        var buff = sock._sQ;
-        var offset = sock._sQlen;
+        const buff = sock._sQ;
+        const offset = sock._sQlen;
 
         buff[offset] = 4;  // msg-type
         buff[offset + 1] = down;
@@ -1622,17 +1613,16 @@ RFB.messages = {
 
     QEMUExtendedKeyEvent: function (sock, keysym, down, keycode) {
         function getRFBkeycode(xt_scancode) {
-            var upperByte = (keycode >> 8);
-            var lowerByte = (keycode & 0x00ff);
+            const upperByte = (keycode >> 8);
+            const lowerByte = (keycode & 0x00ff);
             if (upperByte === 0xe0 && lowerByte < 0x7f) {
-                lowerByte = lowerByte | 0x80;
-                return lowerByte;
+                return lowerByte | 0x80;
             }
             return xt_scancode;
         }
 
-        var buff = sock._sQ;
-        var offset = sock._sQlen;
+        const buff = sock._sQ;
+        const offset = sock._sQlen;
 
         buff[offset] = 255; // msg-type
         buff[offset + 1] = 0; // sub msg-type
@@ -1645,7 +1635,7 @@ RFB.messages = {
         buff[offset + 6] = (keysym >> 8);
         buff[offset + 7] = keysym;
 
-        var RFBkeycode = getRFBkeycode(keycode);
+        const RFBkeycode = getRFBkeycode(keycode);
 
         buff[offset + 8] = (RFBkeycode >> 24);
         buff[offset + 9] = (RFBkeycode >> 16);
@@ -1657,8 +1647,8 @@ RFB.messages = {
     },
 
     pointerEvent: function (sock, x, y, mask) {
-        var buff = sock._sQ;
-        var offset = sock._sQlen;
+        const buff = sock._sQ;
+        const offset = sock._sQlen;
 
         buff[offset] = 5; // msg-type
 
@@ -1676,8 +1666,8 @@ RFB.messages = {
 
     // TODO(directxman12): make this unicode compatible?
     clientCutText: function (sock, text) {
-        var buff = sock._sQ;
-        var offset = sock._sQlen;
+        const buff = sock._sQ;
+        const offset = sock._sQlen;
 
         buff[offset] = 6; // msg-type
 
@@ -1685,7 +1675,7 @@ RFB.messages = {
         buff[offset + 2] = 0; // padding
         buff[offset + 3] = 0; // padding
 
-        let length = text.length;
+        const length = text.length;
 
         buff[offset + 4] = length >> 24;
         buff[offset + 5] = length >> 16;
@@ -1701,16 +1691,14 @@ RFB.messages = {
         let remaining = length;
         while (remaining > 0) {
 
-            let flushSize = Math.min(remaining, (sock._sQbufferSize - sock._sQlen));
+            const flushSize = Math.min(remaining, (sock._sQbufferSize - sock._sQlen));
             if (flushSize <= 0) {
                 this._fail("Clipboard contents could not be sent");
                 break;
             }
 
-            offset = sock._sQlen;
-
             for (let i = 0; i < flushSize; i++) {
-                buff[offset + i] =  text.charCodeAt(textOffset + i);
+                buff[sock._sQlen + i] =  text.charCodeAt(textOffset + i);
             }
 
             sock._sQlen += flushSize;
@@ -1722,8 +1710,8 @@ RFB.messages = {
     },
 
     setDesktopSize: function (sock, width, height, id, flags) {
-        var buff = sock._sQ;
-        var offset = sock._sQlen;
+        const buff = sock._sQ;
+        const offset = sock._sQlen;
 
         buff[offset] = 251;              // msg-type
         buff[offset + 1] = 0;            // padding
@@ -1758,8 +1746,8 @@ RFB.messages = {
     },
 
     clientFence: function (sock, flags, payload) {
-        var buff = sock._sQ;
-        var offset = sock._sQlen;
+        const buff = sock._sQ;
+        const offset = sock._sQlen;
 
         buff[offset] = 248; // msg-type
 
@@ -1772,11 +1760,11 @@ RFB.messages = {
         buff[offset + 6] = flags >> 8;
         buff[offset + 7] = flags;
 
-        var n = payload.length;
+        const n = payload.length;
 
         buff[offset + 8] = n; // length
 
-        for (var i = 0; i < n; i++) {
+        for (let i = 0; i < n; i++) {
             buff[offset + 9 + i] = payload.charCodeAt(i);
         }
 
@@ -1785,8 +1773,8 @@ RFB.messages = {
     },
 
     enableContinuousUpdates: function (sock, enable, x, y, width, height) {
-        var buff = sock._sQ;
-        var offset = sock._sQlen;
+        const buff = sock._sQ;
+        const offset = sock._sQlen;
 
         buff[offset] = 150;             // msg-type
         buff[offset + 1] = enable;      // enable-flag
@@ -1805,10 +1793,10 @@ RFB.messages = {
     },
 
     pixelFormat: function (sock, depth, true_color) {
-        var buff = sock._sQ;
-        var offset = sock._sQlen;
+        const buff = sock._sQ;
+        const offset = sock._sQlen;
 
-        var bpp, bits;
+        let bpp;
 
         if (depth > 16) {
             bpp = 32;
@@ -1818,7 +1806,7 @@ RFB.messages = {
             bpp = 8;
         }
 
-        bits = Math.floor(depth/3);
+        const bits = Math.floor(depth/3);
 
         buff[offset] = 0;  // msg-type
 
@@ -1853,8 +1841,8 @@ RFB.messages = {
     },
 
     clientEncodings: function (sock, encodings) {
-        var buff = sock._sQ;
-        var offset = sock._sQlen;
+        const buff = sock._sQ;
+        const offset = sock._sQlen;
 
         buff[offset] = 2; // msg-type
         buff[offset + 1] = 0; // padding
@@ -1862,9 +1850,9 @@ RFB.messages = {
         buff[offset + 2] = encodings.length >> 8;
         buff[offset + 3] = encodings.length;
 
-        var i, j = offset + 4;
-        for (i = 0; i < encodings.length; i++) {
-            var enc = encodings[i];
+        let j = offset + 4;
+        for (let i = 0; i < encodings.length; i++) {
+            const enc = encodings[i];
             buff[j] = enc >> 24;
             buff[j + 1] = enc >> 16;
             buff[j + 2] = enc >> 8;
@@ -1878,8 +1866,8 @@ RFB.messages = {
     },
 
     fbUpdateRequest: function (sock, incremental, x, y, w, h) {
-        var buff = sock._sQ;
-        var offset = sock._sQlen;
+        const buff = sock._sQ;
+        const offset = sock._sQlen;
 
         if (typeof(x) === "undefined") { x = 0; }
         if (typeof(y) === "undefined") { y = 0; }
@@ -1904,8 +1892,8 @@ RFB.messages = {
     },
 
     xvpOp: function (sock, ver, op) {
-        var buff = sock._sQ;
-        var offset = sock._sQlen;
+        const buff = sock._sQ;
+        const offset = sock._sQlen;
 
         buff[offset] = 250; // msg-type
         buff[offset + 1] = 0; // padding
@@ -1919,8 +1907,8 @@ RFB.messages = {
 };
 
 RFB.genDES = function (password, challenge) {
-    var passwd = [];
-    for (var i = 0; i < password.length; i++) {
+    const passwd = [];
+    for (let i = 0; i < password.length; i++) {
         passwd.push(password.charCodeAt(i));
     }
     return (new DES(passwd)).encrypt(challenge);
@@ -1932,19 +1920,18 @@ RFB.encodingHandlers = {
             this._FBU.lines = this._FBU.height;
         }
 
-        var pixelSize = this._fb_depth == 8 ? 1 : 4;
+        const pixelSize = this._fb_depth == 8 ? 1 : 4;
         this._FBU.bytes = this._FBU.width * pixelSize;  // at least a line
         if (this._sock.rQwait("RAW", this._FBU.bytes)) { return false; }
-        var cur_y = this._FBU.y + (this._FBU.height - this._FBU.lines);
-        var curr_height = Math.min(this._FBU.lines,
+        const cur_y = this._FBU.y + (this._FBU.height - this._FBU.lines);
+        const curr_height = Math.min(this._FBU.lines,
                                    Math.floor(this._sock.rQlen() / (this._FBU.width * pixelSize)));
-        var data = this._sock.get_rQ();
-        var index = this._sock.get_rQi();
+        let data = this._sock.get_rQ();
+        let index = this._sock.get_rQi();
         if (this._fb_depth == 8) {
-            var pixels = this._FBU.width * curr_height
-            var newdata = new Uint8Array(pixels * 4);
-            var i;
-            for (i = 0;i < pixels;i++) {
+            const pixels = this._FBU.width * curr_height
+            const newdata = new Uint8Array(pixels * 4);
+            for (let i = 0; i < pixels; i++) {
                 newdata[i * 4 + 0] = ((data[index + i] >> 0) & 0x3) * 255 / 3;
                 newdata[i * 4 + 1] = ((data[index + i] >> 2) & 0x3) * 255 / 3;
                 newdata[i * 4 + 2] = ((data[index + i] >> 4) & 0x3) * 255 / 3;
@@ -1981,7 +1968,7 @@ RFB.encodingHandlers = {
     },
 
     RRE: function () {
-        var color;
+        let color;
         if (this._FBU.subrects === 0) {
             this._FBU.bytes = 4 + 4;
             if (this._sock.rQwait("RRE", 4 + 4)) { return false; }
@@ -1992,16 +1979,16 @@ RFB.encodingHandlers = {
 
         while (this._FBU.subrects > 0 && this._sock.rQlen() >= (4 + 8)) {
             color = this._sock.rQshiftBytes(4);
-            var x = this._sock.rQshift16();
-            var y = this._sock.rQshift16();
-            var width = this._sock.rQshift16();
-            var height = this._sock.rQshift16();
+            const x = this._sock.rQshift16();
+            const y = this._sock.rQshift16();
+            const width = this._sock.rQshift16();
+            const height = this._sock.rQshift16();
             this._display.fillRect(this._FBU.x + x, this._FBU.y + y, width, height, color);
             this._FBU.subrects--;
         }
 
         if (this._FBU.subrects > 0) {
-            var chunk = Math.min(this._rre_chunk_sz, this._FBU.subrects);
+            const chunk = Math.min(this._rre_chunk_sz, this._FBU.subrects);
             this._FBU.bytes = (4 + 8) * chunk;
         } else {
             this._FBU.rects--;
@@ -2012,8 +1999,8 @@ RFB.encodingHandlers = {
     },
 
     HEXTILE: function () {
-        var rQ = this._sock.get_rQ();
-        var rQi = this._sock.get_rQi();
+        const rQ = this._sock.get_rQ();
+        let rQi = this._sock.get_rQi();
 
         if (this._FBU.tiles === 0) {
             this._FBU.tiles_x = Math.ceil(this._FBU.width / 16);
@@ -2025,21 +2012,21 @@ RFB.encodingHandlers = {
         while (this._FBU.tiles > 0) {
             this._FBU.bytes = 1;
             if (this._sock.rQwait("HEXTILE subencoding", this._FBU.bytes)) { return false; }
-            var subencoding = rQ[rQi];  // Peek
+            const subencoding = rQ[rQi];  // Peek
             if (subencoding > 30) {  // Raw
                 this._fail("Illegal hextile subencoding (subencoding: " +
                            subencoding + ")");
                 return false;
             }
 
-            var subrects = 0;
-            var curr_tile = this._FBU.total_tiles - this._FBU.tiles;
-            var tile_x = curr_tile % this._FBU.tiles_x;
-            var tile_y = Math.floor(curr_tile / this._FBU.tiles_x);
-            var x = this._FBU.x + tile_x * 16;
-            var y = this._FBU.y + tile_y * 16;
-            var w = Math.min(16, (this._FBU.x + this._FBU.width) - x);
-            var h = Math.min(16, (this._FBU.y + this._FBU.height) - y);
+            let subrects = 0;
+            const curr_tile = this._FBU.total_tiles - this._FBU.tiles;
+            const tile_x = curr_tile % this._FBU.tiles_x;
+            const tile_y = Math.floor(curr_tile / this._FBU.tiles_x);
+            const x = this._FBU.x + tile_x * 16;
+            const y = this._FBU.y + tile_y * 16;
+            const w = Math.min(16, (this._FBU.x + this._FBU.width) - x);
+            const h = Math.min(16, (this._FBU.y + this._FBU.height) - y);
 
             // Figure out how much we are expecting
             if (subencoding & 0x01) {  // Raw
@@ -2093,23 +2080,23 @@ RFB.encodingHandlers = {
                     subrects = rQ[rQi];
                     rQi++;
 
-                    for (var s = 0; s < subrects; s++) {
-                        var color;
+                    for (let s = 0; s < subrects; s++) {
+                        let color;
                         if (this._FBU.subencoding & 0x10) {  // SubrectsColoured
                             color = [rQ[rQi], rQ[rQi + 1], rQ[rQi + 2], rQ[rQi + 3]];
                             rQi += 4;
                         } else {
                             color = this._FBU.foreground;
                         }
-                        var xy = rQ[rQi];
+                        const xy = rQ[rQi];
                         rQi++;
-                        var sx = (xy >> 4);
-                        var sy = (xy & 0x0f);
+                        const sx = (xy >> 4);
+                        const sy = (xy & 0x0f);
 
-                        var wh = rQ[rQi];
+                        const wh = rQ[rQi];
                         rQi++;
-                        var sw = (wh >> 4) + 1;
-                        var sh = (wh & 0x0f) + 1;
+                        const sw = (wh >> 4) + 1;
+                        const sh = (wh & 0x0f) + 1;
 
                         this._display.subTile(sx, sy, sw, sh, color);
                     }
@@ -2133,18 +2120,18 @@ RFB.encodingHandlers = {
         this._FBU.bytes = 1;  // compression-control byte
         if (this._sock.rQwait("TIGHT compression-control", this._FBU.bytes)) { return false; }
 
-        var resetStreams = 0;
-        var streamId = -1;
-        var decompress = function (data, expected) {
-            for (var i = 0; i < 4; i++) {
+        let resetStreams = 0;
+        let streamId = -1;
+        const decompress = function (data, expected) {
+            for (let i = 0; i < 4; i++) {
                 if ((resetStreams >> i) & 1) {
                     this._FBU.zlibs[i].reset();
                     Log.Info("Reset zlib stream " + i);
                 }
             }
 
-            //var uncompressed = this._FBU.zlibs[streamId].uncompress(data, 0);
-            var uncompressed = this._FBU.zlibs[streamId].inflate(data, true, expected);
+            //const uncompressed = this._FBU.zlibs[streamId].uncompress(data, 0);
+            const uncompressed = this._FBU.zlibs[streamId].inflate(data, true, expected);
             /*if (uncompressed.status !== 0) {
                 Log.Error("Invalid data in zlib stream");
             }*/
@@ -2153,18 +2140,18 @@ RFB.encodingHandlers = {
             return uncompressed;
         }.bind(this);
 
-        var indexedToRGBX2Color = function (data, palette, width, height) {
+        const indexedToRGBX2Color = function (data, palette, width, height) {
             // Convert indexed (palette based) image data to RGB
             // TODO: reduce number of calculations inside loop
-            var dest = this._destBuff;
-            var w = Math.floor((width + 7) / 8);
-            var w1 = Math.floor(width / 8);
-
-            /*for (var y = 0; y < height; y++) {
-                var b, x, dp, sp;
-                var yoffset = y * width;
-                var ybitoffset = y * w;
-                var xoffset, targetbyte;
+            const dest = this._destBuff;
+            const w = Math.floor((width + 7) / 8);
+            const w1 = Math.floor(width / 8);
+
+            /*for (let y = 0; y < height; y++) {
+                let b, x, dp, sp;
+                const yoffset = y * width;
+                const ybitoffset = y * w;
+                let xoffset, targetbyte;
                 for (x = 0; x < w1; x++) {
                     xoffset = yoffset + x * 8;
                     targetbyte = data[ybitoffset + x];
@@ -2188,10 +2175,10 @@ RFB.encodingHandlers = {
                 }
             }*/
 
-            for (var y = 0; y < height; y++) {
-                var b, x, dp, sp;
+            for (let y = 0; y < height; y++) {
+                let dp, sp, x;
                 for (x = 0; x < w1; x++) {
-                    for (b = 7; b >= 0; b--) {
+                    for (let b = 7; b >= 0; b--) {
                         dp = (y * width + x * 8 + 7 - b) * 4;
                         sp = (data[y * w + x] >> b & 1) * 3;
                         dest[dp] = palette[sp];
@@ -2201,7 +2188,7 @@ RFB.encodingHandlers = {
                     }
                 }
 
-                for (b = 7; b >= 8 - width % 8; b--) {
+                for (let b = 7; b >= 8 - width % 8; b--) {
                     dp = (y * width + x * 8 + 7 - b) * 4;
                     sp = (data[y * w + x] >> b & 1) * 3;
                     dest[dp] = palette[sp];
@@ -2214,12 +2201,12 @@ RFB.encodingHandlers = {
             return dest;
         }.bind(this);
 
-        var indexedToRGBX = function (data, palette, width, height) {
+        const indexedToRGBX = function (data, palette, width, height) {
             // Convert indexed (palette based) image data to RGB
-            var dest = this._destBuff;
-            var total = width * height * 4;
-            for (var i = 0, j = 0; i < total; i += 4, j++) {
-                var sp = data[j] * 3;
+            const dest = this._destBuff;
+            const total = width * height * 4;
+            for (let i = 0, j = 0; i < total; i += 4, j++) {
+                const sp = data[j] * 3;
                 dest[i] = palette[sp];
                 dest[i + 1] = palette[sp + 1];
                 dest[i + 2] = palette[sp + 2];
@@ -2229,20 +2216,20 @@ RFB.encodingHandlers = {
             return dest;
         }.bind(this);
 
-        var rQi = this._sock.get_rQi();
-        var rQ = this._sock.rQwhole();
-        var cmode, data;
-        var cl_header, cl_data;
+        const rQi = this._sock.get_rQi();
+        const rQ = this._sock.rQwhole();
+        let cmode, data;
+        let cl_header, cl_data;
 
-        var handlePalette = function () {
-            var numColors = rQ[rQi + 2] + 1;
-            var paletteSize = numColors * 3;
+        const handlePalette = function () {
+            const numColors = rQ[rQi + 2] + 1;
+            const paletteSize = numColors * 3;
             this._FBU.bytes += paletteSize;
             if (this._sock.rQwait("TIGHT palette " + cmode, this._FBU.bytes)) { return false; }
 
-            var bpp = (numColors <= 2) ? 1 : 8;
-            var rowSize = Math.floor((this._FBU.width * bpp + 7) / 8);
-            var raw = false;
+            const bpp = (numColors <= 2) ? 1 : 8;
+            const rowSize = Math.floor((this._FBU.width * bpp + 7) / 8);
+            let raw = false;
             if (rowSize * this._FBU.height < 12) {
                 raw = true;
                 cl_header = 0;
@@ -2250,7 +2237,7 @@ RFB.encodingHandlers = {
                 //clength = [0, rowSize * this._FBU.height];
             } else {
                 // begin inline getTightCLength (returning two-item arrays is bad for performance with GC)
-                var cl_offset = rQi + 3 + paletteSize;
+                const cl_offset = rQi + 3 + paletteSize;
                 cl_header = 1;
                 cl_data = 0;
                 cl_data += rQ[cl_offset] & 0x7f;
@@ -2270,7 +2257,7 @@ RFB.encodingHandlers = {
 
             // Shift ctl, filter id, num colors, palette entries, and clength off
             this._sock.rQskipBytes(3);
-            //var palette = this._sock.rQshiftBytes(paletteSize);
+            //const palette = this._sock.rQshiftBytes(paletteSize);
             this._sock.rQshiftTo(this._paletteBuff, paletteSize);
             this._sock.rQskipBytes(cl_header);
 
@@ -2281,29 +2268,29 @@ RFB.encodingHandlers = {
             }
 
             // Convert indexed (palette based) image data to RGB
-            var rgbx;
+            let rgbx;
             if (numColors == 2) {
                 rgbx = indexedToRGBX2Color(data, this._paletteBuff, this._FBU.width, this._FBU.height);
-                this._display.blitRgbxImage(this._FBU.x, this._FBU.y, this._FBU.width, this._FBU.height, rgbx, 0, false);
             } else {
                 rgbx = indexedToRGBX(data, this._paletteBuff, this._FBU.width, this._FBU.height);
-                this._display.blitRgbxImage(this._FBU.x, this._FBU.y, this._FBU.width, this._FBU.height, rgbx, 0, false);
             }
 
+            this._display.blitRgbxImage(this._FBU.x, this._FBU.y, this._FBU.width, this._FBU.height, rgbx, 0, false);
+
 
             return true;
         }.bind(this);
 
-        var handleCopy = function () {
-            var raw = false;
-            var uncompressedSize = this._FBU.width * this._FBU.height * 3;
+        const handleCopy = function () {
+            let raw = false;
+            const uncompressedSize = this._FBU.width * this._FBU.height * 3;
             if (uncompressedSize < 12) {
                 raw = true;
                 cl_header = 0;
                 cl_data = uncompressedSize;
             } else {
                 // begin inline getTightCLength (returning two-item arrays is for peformance with GC)
-                var cl_offset = rQi + 1;
+                const cl_offset = rQi + 1;
                 cl_header = 1;
                 cl_data = 0;
                 cl_data += rQ[cl_offset] & 0x7f;
@@ -2334,7 +2321,7 @@ RFB.encodingHandlers = {
             return true;
         }.bind(this);
 
-        var ctl = this._sock.rQpeek8();
+        let ctl = this._sock.rQpeek8();
 
         // Keep tight reset bits
         resetStreams = ctl & 0xF;
@@ -2379,6 +2366,7 @@ RFB.encodingHandlers = {
         if (this._sock.rQwait("TIGHT " + cmode, this._FBU.bytes)) { return false; }
 
         // Determine FBU.bytes
+        let cl_offset, filterId;
         switch (cmode) {
             case "fill":
                 // skip ctl byte
@@ -2388,7 +2376,7 @@ RFB.encodingHandlers = {
             case "png":
             case "jpeg":
                 // begin inline getTightCLength (returning two-item arrays is for peformance with GC)
-                var cl_offset = rQi + 1;
+                cl_offset = rQi + 1;
                 cl_header = 1;
                 cl_data = 0;
                 cl_data += rQ[cl_offset] & 0x7f;
@@ -2410,7 +2398,7 @@ RFB.encodingHandlers = {
                 this._display.imageRect(this._FBU.x, this._FBU.y, "image/" + cmode, data);
                 break;
             case "filter":
-                var filterId = rQ[rQi + 1];
+                filterId = rQ[rQi + 1];
                 if (filterId === 1) {
                     if (!handlePalette()) { return false; }
                 } else {
@@ -2441,7 +2429,7 @@ RFB.encodingHandlers = {
         this._FBU.bytes = 1;
         if (this._sock.rQwait("ExtendedDesktopSize", this._FBU.bytes)) { return false; }
 
-        var firstUpdate = !this._supportsSetDesktopSize;
+        const firstUpdate = !this._supportsSetDesktopSize;
         this._supportsSetDesktopSize = true;
 
         // Normally we only apply the current resize mode after a
@@ -2452,7 +2440,7 @@ RFB.encodingHandlers = {
             this._requestRemoteResize();
         }
 
-        var number_of_screens = this._sock.rQpeek8();
+        const number_of_screens = this._sock.rQpeek8();
 
         this._FBU.bytes = 4 + (number_of_screens * 16);
         if (this._sock.rQwait("ExtendedDesktopSize", this._FBU.bytes)) { return false; }
@@ -2460,7 +2448,7 @@ RFB.encodingHandlers = {
         this._sock.rQskipBytes(1);  // number-of-screens
         this._sock.rQskipBytes(3);  // padding
 
-        for (var i = 0; i < number_of_screens; i += 1) {
+        for (let i = 0; i < number_of_screens; i += 1) {
             // Save the id and flags of the first screen
             if (i === 0) {
                 this._screen_id = this._sock.rQshiftBytes(4);    // id
@@ -2484,7 +2472,7 @@ RFB.encodingHandlers = {
 
         // We need to handle errors when we requested the resize.
         if (this._FBU.x === 1 && this._FBU.y !== 0) {
-            var msg = "";
+            let msg = "";
             // The y-position indicates the status code from the server
             switch (this._FBU.y) {
             case 1:
@@ -2520,13 +2508,13 @@ RFB.encodingHandlers = {
 
     Cursor: function () {
         Log.Debug(">> set_cursor");
-        var x = this._FBU.x;  // hotspot-x
-        var y = this._FBU.y;  // hotspot-y
-        var w = this._FBU.width;
-        var h = this._FBU.height;
+        const x = this._FBU.x;  // hotspot-x
+        const y = this._FBU.y;  // hotspot-y
+        const w = this._FBU.width;
+        const h = this._FBU.height;
 
-        var pixelslength = w * h * 4;
-        var masklength = Math.floor((w + 7) / 8) * h;
+        const pixelslength = w * h * 4;
+        const masklength = Math.floor((w + 7) / 8) * h;
 
         this._FBU.bytes = pixelslength + masklength;
         if (this._sock.rQwait("cursor encoding", this._FBU.bytes)) { return false; }
@@ -2547,7 +2535,7 @@ RFB.encodingHandlers = {
 
         // Old Safari doesn't support creating keyboard events
         try {
-            var keyboardEvent = document.createEvent("keyboardEvent");
+            const keyboardEvent = document.createEvent("keyboardEvent");
             if (keyboardEvent.code !== undefined) {
                 this._qemuExtKeyEventSupported = true;
             }
diff --git a/core/util/browser.js b/core/util/browser.js
index 2b44fb2..80551d4 100644
--- a/core/util/browser.js
+++ b/core/util/browser.js
@@ -9,7 +9,7 @@
 import * as Log from './logging.js';
 
 // Touch detection
-export var isTouchDevice = ('ontouchstart' in document.documentElement) ||
+export let isTouchDevice = ('ontouchstart' in document.documentElement) ||
                                  // requried for Chrome debugger
                                  (document.ontouchstart !== undefined) ||
                                  // required for MS Surface
@@ -20,12 +20,12 @@ window.addEventListener('touchstart', function onFirstTouch() {
     window.removeEventListener('touchstart', onFirstTouch, false);
 }, false);
 
-var _cursor_uris_supported = null;
+let _cursor_uris_supported = null;
 
 export function supportsCursorURIs () {
     if (_cursor_uris_supported === null) {
         try {
-            var target = document.createElement('canvas');
+            const target = document.createElement('canvas');
             target.style.cursor = 'url("data:image/x-icon;base64,AAACAAEACAgAAAIAAgA4AQAAFgAAACgAAAAIAAAAEAAAAAEAIAAAAAAAEAAAAAAAAAAAAAAAAAAAAAAAAAD/////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////AAAAAAAAAAAAAAAAAAAAAA==") 2 2, default';
 
             if (target.style.cursor) {
diff --git a/core/util/events.js b/core/util/events.js
index 270a558..66527fa 100644
--- a/core/util/events.js
+++ b/core/util/events.js
@@ -20,14 +20,14 @@ export function stopEvent (e) {
 }
 
 // Emulate Element.setCapture() when not supported
-var _captureRecursion = false;
-var _captureElem = null;
+let _captureRecursion = false;
+let _captureElem = null;
 function _captureProxy(e) {
     // Recursion protection as we'll see our own event
     if (_captureRecursion) return;
 
     // Clone the event as we cannot dispatch an already dispatched event
-    var newEv = new e.constructor(e.type, e);
+    const newEv = new e.constructor(e.type, e);
 
     _captureRecursion = true;
     _captureElem.dispatchEvent(newEv);
@@ -49,12 +49,13 @@ function _captureProxy(e) {
 
 // Follow cursor style of target element
 function _captureElemChanged() {
-    var captureElem = document.getElementById("noVNC_mouse_capture_elem");
+    const captureElem = document.getElementById("noVNC_mouse_capture_elem");
     captureElem.style.cursor = window.getComputedStyle(_captureElem).cursor;
 }
-var _captureObserver = new MutationObserver(_captureElemChanged);
 
-var _captureIndex = 0;
+const _captureObserver = new MutationObserver(_captureElemChanged);
+
+let _captureIndex = 0;
 
 export function setCapture (elem) {
     if (elem.setCapture) {
@@ -69,7 +70,7 @@ export function setCapture (elem) {
         // called multiple times without coordination
         releaseCapture();
 
-        var captureElem = document.getElementById("noVNC_mouse_capture_elem");
+        let captureElem = document.getElementById("noVNC_mouse_capture_elem");
 
         if (captureElem === null) {
             captureElem = document.createElement("div");
@@ -129,7 +130,7 @@ export function releaseCapture () {
 
         _captureObserver.disconnect();
 
-        var captureElem = document.getElementById("noVNC_mouse_capture_elem");
+        const captureElem = document.getElementById("noVNC_mouse_capture_elem");
         captureElem.style.display = "none";
 
         window.removeEventListener('mousemove', _captureProxy);
diff --git a/core/util/eventtarget.js b/core/util/eventtarget.js
index 61bc7a1..2472319 100644
--- a/core/util/eventtarget.js
+++ b/core/util/eventtarget.js
@@ -6,7 +6,7 @@
  * See README.md for usage and integration instructions.
  */
 
-var EventTargetMixin = {
+const EventTargetMixin = {
     _listeners: null,
 
    addEventListener: function(type, callback) {
diff --git a/core/util/logging.js b/core/util/logging.js
index 3b60f69..5669ad2 100644
--- a/core/util/logging.js
+++ b/core/util/logging.js
@@ -10,12 +10,12 @@
  * Logging/debug routines
  */
 
-var _log_level = 'warn';
+let _log_level = 'warn';
 
-var Debug = function (msg) {};
-var Info = function (msg) {};
-var Warn = function (msg) {};
-var Error = function (msg) {};
+let Debug = function (msg) {};
+let Info = function (msg) {};
+let Warn = function (msg) {};
+let Error = function (msg) {};
 
 export function init_logging (level) {
     if (typeof level === 'undefined') {
diff --git a/core/util/polyfill.js b/core/util/polyfill.js
index 8c600e6..c991aa2 100644
--- a/core/util/polyfill.js
+++ b/core/util/polyfill.js
@@ -16,13 +16,13 @@ if (typeof Object.assign != 'function') {
                 throw new TypeError('Cannot convert undefined or null to object');
             }
 
-            var to = Object(target);
+            const to = Object(target);
 
-            for (var index = 1; index < arguments.length; index++) {
-                var nextSource = arguments[index];
+            for (let index = 1; index < arguments.length; index++) {
+                const nextSource = arguments[index];
 
                 if (nextSource != null) { // Skip over if undefined or null
-                    for (var nextKey in nextSource) {
+                    for (let nextKey in nextSource) {
                         // Avoid bugs when hasOwnProperty is shadowed
                         if (Object.prototype.hasOwnProperty.call(nextSource, nextKey)) {
                             to[nextKey] = nextSource[nextKey];
@@ -41,7 +41,7 @@ if (typeof Object.assign != 'function') {
 (function () {
     function CustomEvent ( event, params ) {
         params = params || { bubbles: false, cancelable: false, detail: undefined };
-        var evt = document.createEvent( 'CustomEvent' );
+        const evt = document.createEvent( 'CustomEvent' );
         evt.initCustomEvent( event, params.bubbles, params.cancelable, params.detail );
         return evt;
     }
diff --git a/core/websock.js b/core/websock.js
index a5643dd..f07a7be 100644
--- a/core/websock.js
+++ b/core/websock.js
@@ -42,15 +42,15 @@ export default function Websock() {
 // this has performance issues in some versions Chromium, and
 // doesn't gain a tremendous amount of performance increase in Firefox
 // at the moment.  It may be valuable to turn it on in the future.
-var ENABLE_COPYWITHIN = false;
+const ENABLE_COPYWITHIN = false;
 
-var MAX_RQ_GROW_SIZE = 40 * 1024 * 1024;  // 40 MiB
+const MAX_RQ_GROW_SIZE = 40 * 1024 * 1024;  // 40 MiB
 
-var typedArrayToString = (function () {
+const typedArrayToString = (function () {
     // This is only for PhantomJS, which doesn't like apply-ing
     // with Typed Arrays
     try {
-        var arr = new Uint8Array([1, 2, 3]);
+        const arr = new Uint8Array([1, 2, 3]);
         String.fromCharCode.apply(null, arr);
         return function (a) { return String.fromCharCode.apply(null, a); };
     } catch (ex) {
@@ -115,7 +115,7 @@ Websock.prototype = {
 
     rQshiftStr: function (len) {
         if (typeof(len) === 'undefined') { len = this.rQlen(); }
-        var arr = new Uint8Array(this._rQ.buffer, this._rQi, len);
+        const arr = new Uint8Array(this._rQ.buffer, this._rQi, len);
         this._rQi += len;
         return typedArrayToString(arr);
     },
@@ -149,7 +149,7 @@ Websock.prototype = {
     // to be available in the receive queue. Return true if we need to
     // wait (and possibly print a debug message), otherwise false.
     rQwait: function (msg, num, goback) {
-        var rQlen = this._rQlen - this._rQi; // Skip rQlen() function call
+        const rQlen = this._rQlen - this._rQi; // Skip rQlen() function call
         if (rQlen < num) {
             if (goback) {
                 if (this._rQi < goback) {
@@ -251,7 +251,7 @@ Websock.prototype = {
     },
 
     _expand_compact_rQ: function (min_fit) {
-        var resizeNeeded = min_fit || this._rQlen - this._rQi > this._rQbufferSize / 2;
+        const resizeNeeded = min_fit || this._rQlen - this._rQi > this._rQbufferSize / 2;
         if (resizeNeeded) {
             if (!min_fit) {
                 // just double the size if we need to do compaction
@@ -271,7 +271,7 @@ Websock.prototype = {
         }
 
         if (resizeNeeded) {
-            var old_rQbuffer = this._rQ.buffer;
+            const old_rQbuffer = this._rQ.buffer;
             this._rQmax = this._rQbufferSize / 8;
             this._rQ = new Uint8Array(this._rQbufferSize);
             this._rQ.set(new Uint8Array(old_rQbuffer, this._rQi));
@@ -289,7 +289,7 @@ Websock.prototype = {
 
     _decode_message: function (data) {
         // push arraybuffer values onto the end
-        var u8 = new Uint8Array(data);
+        const u8 = new Uint8Array(data);
         if (u8.length > this._rQbufferSize - this._rQlen) {
             this._expand_compact_rQ(u8.length);
         }
diff --git a/docs/API.md b/docs/API.md
index c5923e3..a81da5c 100644
--- a/docs/API.md
+++ b/docs/API.md
@@ -137,7 +137,7 @@ connection to a specified VNC server.
 
 ##### Syntax
 
-    var rfb = new RFB( target, url [, options] );
+    let rfb = new RFB( target, url [, options] );
 
 ###### Parameters
 
diff --git a/po/po2js b/po/po2js
index 15304c9..6531716 100755
--- a/po/po2js
+++ b/po/po2js
@@ -17,11 +17,11 @@
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  */
 
-var getopt = require('node-getopt');
-var fs = require('fs');
-var po2json = require("po2json");
+const getopt = require('node-getopt');
+const fs = require('fs');
+const po2json = require("po2json");
 
-opt = getopt.create([
+const opt = getopt.create([
   ['h' , 'help'                , 'display this help'],
 ]).bindHelp().parseSystem();
 
@@ -30,14 +30,14 @@ if (opt.argv.length != 2) {
   process.exit(1);
 }
 
-var data = po2json.parseFileSync(opt.argv[0]);
+const data = po2json.parseFileSync(opt.argv[0]);
 
-var bodyPart = Object.keys(data).filter((msgid) => msgid !== "").map((msgid) => {
+const bodyPart = Object.keys(data).filter((msgid) => msgid !== "").map((msgid) => {
     if (msgid === "") return;
-    var msgstr = data[msgid][1];
+    const msgstr = data[msgid][1];
     return "    " + JSON.stringify(msgid) + ": " + JSON.stringify(msgstr);
 }).join(",\n");
 
-var output = "{\n" + bodyPart + "\n}";
+const output = "{\n" + bodyPart + "\n}";
 
 fs.writeFileSync(opt.argv[1], output);
diff --git a/po/xgettext-html b/po/xgettext-html
index 231de44..87638cb 100755
--- a/po/xgettext-html
+++ b/po/xgettext-html
@@ -5,17 +5,16 @@
  * Licensed under MPL 2.0 (see LICENSE.txt)
  */
 
-var getopt = require('node-getopt');
+const getopt = require('node-getopt');
+const jsdom = require("jsdom");
+const fs = require("fs");
 
-var jsdom = require("jsdom");
-var fs = require("fs");
-
-opt = getopt.create([
+const opt = getopt.create([
     ['o' , 'output=FILE'      , 'write output to specified file'],
     ['h' , 'help'             , 'display this help'],
 ]).bindHelp().parseSystem();
 
-var strings = {};
+const strings = {};
 
 function addString(str, location) {
     if (str.length == 0) {
@@ -74,7 +73,7 @@ function process(elem, locator, enabled) {
         }
     }
 
-    for (var i = 0;i < elem.childNodes.length;i++) {
+    for (let i = 0; i < elem.childNodes.length; i++) {
         node = elem.childNodes[i];
         if (node.nodeType === node.ELEMENT_NODE) {
             process(node, locator, enabled);
@@ -84,24 +83,22 @@ function process(elem, locator, enabled) {
     }
 }
 
-for (var i = 0;i < opt.argv.length;i++) {
-    var file;
-
-    fn = opt.argv[i];
-    file = fs.readFileSync(fn, "utf8");
-    dom = new jsdom.JSDOM(file, { includeNodeLocations: true });
-    body = dom.window.document.body;
+for (let i = 0; i < opt.argv.length; i++) {
+    const fn = opt.argv[i];
+    const file = fs.readFileSync(fn, "utf8");
+    const dom = new jsdom.JSDOM(file, { includeNodeLocations: true });
+    const body = dom.window.document.body;
 
-    locator = function (elem) {
-        offset = dom.nodeLocation(elem).startOffset;
-        line = file.slice(0, offset).split("\n").length;
+    const locator = function (elem) {
+        const offset = dom.nodeLocation(elem).startOffset;
+        const line = file.slice(0, offset).split("\n").length;
         return fn + ":" + line;
     };
 
     process(body, locator, true);
 }
 
-var output = "";
+let output = "";
 
 for (str in strings) {
     output += "#:";
diff --git a/tests/assertions.js b/tests/assertions.js
index 4f1cdc1..a336a7a 100644
--- a/tests/assertions.js
+++ b/tests/assertions.js
@@ -5,15 +5,15 @@ chai.use(sinonChai);
 // noVNC specific assertions
 chai.use(function (_chai, utils) {
     _chai.Assertion.addMethod('displayed', function (target_data) {
-        var obj = this._obj;
-        var ctx = obj._target.getContext('2d');
-        var data_cl = ctx.getImageData(0, 0, obj._target.width, obj._target.height).data;
+        const obj = this._obj;
+        const ctx = obj._target.getContext('2d');
+        const data_cl = ctx.getImageData(0, 0, obj._target.width, obj._target.height).data;
         // NB(directxman12): PhantomJS 1.x doesn't implement Uint8ClampedArray, so work around that
-        var data = new Uint8Array(data_cl);
-        var len = data_cl.length;
+        const data = new Uint8Array(data_cl);
+        const len = data_cl.length;
         new chai.Assertion(len).to.be.equal(target_data.length, "unexpected display size");
-        var same = true;
-        for (var i = 0; i < len; i++) {
+        let same = true;
+        for (let i = 0; i < len; i++) {
             if (data[i] != target_data[i]) {
                 same = false;
                 break;
@@ -31,19 +31,19 @@ chai.use(function (_chai, utils) {
     });
 
     _chai.Assertion.addMethod('sent', function (target_data) {
-        var obj = this._obj;
+        const obj = this._obj;
         obj.inspect = function () {
-            var res = { _websocket: obj._websocket, rQi: obj._rQi, _rQ: new Uint8Array(obj._rQ.buffer, 0, obj._rQlen),
+            const res = { _websocket: obj._websocket, rQi: obj._rQi, _rQ: new Uint8Array(obj._rQ.buffer, 0, obj._rQlen),
                         _sQ: new Uint8Array(obj._sQ.buffer, 0, obj._sQlen) };
             res.prototype = obj;
             return res;
         };
-        var data = obj._websocket._get_sent_data();
-        var same = true;
+        const data = obj._websocket._get_sent_data();
+        let same = true;
         if (data.length != target_data.length) {
             same = false;
         } else {
-            for (var i = 0; i < data.length; i++) {
+            for (let i = 0; i < data.length; i++) {
                 if (data[i] != target_data[i]) {
                     same = false;
                     break;
@@ -68,13 +68,12 @@ chai.use(function (_chai, utils) {
     _chai.Assertion.overwriteMethod('equal', function (_super) {
         return function assertArrayEqual(target) {
             if (utils.flag(this, 'array')) {
-                var obj = this._obj;
+                const obj = this._obj;
 
-                var i;
-                var same = true;
+                let same = true;
 
                 if (utils.flag(this, 'deep')) {
-                    for (i = 0; i < obj.length; i++) {
+                    for (let i = 0; i < obj.length; i++) {
                         if (!utils.eql(obj[i], target[i])) {
                             same = false;
                             break;
@@ -86,7 +85,7 @@ chai.use(function (_chai, utils) {
                         "expected #{this} not to have elements deeply equal to #{exp}",
                         Array.prototype.slice.call(target));
                 } else {
-                    for (i = 0; i < obj.length; i++) {
+                    for (let i = 0; i < obj.length; i++) {
                         if (obj[i] != target[i]) {
                             same = false;
                             break;
diff --git a/tests/fake.websocket.js b/tests/fake.websocket.js
index de3fb30..2e28494 100644
--- a/tests/fake.websocket.js
+++ b/tests/fake.websocket.js
@@ -2,10 +2,10 @@ import Base64 from '../core/base64.js';
 
 // PhantomJS can't create Event objects directly, so we need to use this
 function make_event(name, props) {
-    var evt = document.createEvent('Event');
+    const evt = document.createEvent('Event');
     evt.initEvent(name, true, true);
     if (props) {
-        for (var prop in props) {
+        for (let prop in props) {
             evt[prop] = props[prop];
         }
     }
@@ -50,7 +50,7 @@ FakeWebSocket.prototype = {
     },
 
     _get_sent_data: function () {
-        var res = new Uint8Array(this._send_queue.buffer, 0, this.bufferedAmount);
+        const res = new Uint8Array(this._send_queue.buffer, 0, this.bufferedAmount);
         this.bufferedAmount = 0;
         return res;
     },
@@ -76,7 +76,7 @@ FakeWebSocket.__is_fake = true;
 
 FakeWebSocket.replace = function () {
     if (!WebSocket.__is_fake) {
-        var real_version = WebSocket;
+        const real_version = WebSocket;
         // eslint-disable-next-line no-global-assign
         WebSocket = FakeWebSocket;
         FakeWebSocket.__real_version = real_version;
diff --git a/tests/karma-test-main.js b/tests/karma-test-main.js
index 657e312..334b771 100644
--- a/tests/karma-test-main.js
+++ b/tests/karma-test-main.js
@@ -1,6 +1,6 @@
-var TEST_REGEXP = /test\..*\.js/;
-var allTestFiles = [];
-var extraFiles = ['/base/tests/assertions.js'];
+const TEST_REGEXP = /test\..*\.js/;
+const allTestFiles = [];
+const extraFiles = ['/base/tests/assertions.js'];
 
 Object.keys(window.__karma__.files).forEach(function (file) {
     if (TEST_REGEXP.test(file)) {
diff --git a/tests/playback-ui.js b/tests/playback-ui.js
index 84683cf..1565b8f 100644
--- a/tests/playback-ui.js
+++ b/tests/playback-ui.js
@@ -3,11 +3,11 @@
 import * as WebUtil from '../app/webutil.js';
 import RecordingPlayer from './playback.js';
 
-var frames = null;
-var encoding = null;
+let frames = null;
+let encoding = null;
 
 function message(str) {
-    var cell = document.getElementById('messages');
+    const cell = document.getElementById('messages');
     cell.textContent += str + "\n";
     cell.scrollTop = cell.scrollHeight;
 }
@@ -22,7 +22,7 @@ function loadFile() {
     message("Loading " + fname);
 
     return new Promise(function (resolve, reject) {
-        var script = document.createElement("script");
+        const script = document.createElement("script");
         script.onload = resolve;
         script.onerror = reject;
         document.body.appendChild(script);
@@ -31,10 +31,10 @@ function loadFile() {
 }
 
 function enableUI() {
-    var iterations = WebUtil.getQueryVar('iterations', 3);
+    const iterations = WebUtil.getQueryVar('iterations', 3);
     document.getElementById('iterations').value = iterations;
 
-    var mode = WebUtil.getQueryVar('mode', 3);
+    const mode = WebUtil.getQueryVar('mode', 3);
     if (mode === 'realtime') {
         document.getElementById('mode2').checked = true;
     } else {
@@ -121,7 +121,7 @@ IterationPlayer.prototype = {
             this._state = 'failed';
         }
 
-        var evt = new Event('rfbdisconnected');
+        const evt = new Event('rfbdisconnected');
         evt.clean = clean;
         evt.frame = frame;
         evt.iteration = this._iteration;
@@ -136,7 +136,7 @@ function start() {
 
     const iterations = document.getElementById('iterations').value;
 
-    var mode;
+    let mode;
 
     if (document.getElementById('mode1').checked) {
         message(`Starting performance playback (fullspeed) [${iterations} iteration(s)]`);
diff --git a/tests/playback.js b/tests/playback.js
index 7c9d7ff..cbeb20f 100644
--- a/tests/playback.js
+++ b/tests/playback.js
@@ -9,12 +9,12 @@ import * as Log from '../core/util/logging.js';
 import Base64 from '../core/base64.js';
 
 // Immediate polyfill
-if (setImmediate === undefined) {
-    var _immediateIdCounter = 1;
-    var _immediateFuncs = {};
+if (window.setImmediate === undefined) {
+    let _immediateIdCounter = 1;
+    const _immediateFuncs = {};
 
-    var setImmediate = function (func) {
-        var index = _immediateIdCounter++;
+    window.setImmediate = function (func) {
+        const index = _immediateIdCounter++;
         _immediateFuncs[index] = func;
         window.postMessage("noVNC immediate trigger:" + index, "*");
         return index;
@@ -24,15 +24,15 @@ if (setImmediate === undefined) {
         _immediateFuncs[id];
     };
 
-    var _onMessage = function (event) {
+    const _onMessage = function (event) {
         if ((typeof event.data !== "string") ||
             (event.data.indexOf("noVNC immediate trigger:") !== 0)) {
             return;
         }
 
-        var index = event.data.slice("noVNC immediate trigger:".length);
+        const index = event.data.slice("noVNC immediate trigger:".length);
 
-        var callback = _immediateFuncs[index];
+        const callback = _immediateFuncs[index];
         if (callback === undefined) {
             return;
         }
@@ -51,8 +51,8 @@ export default function RecordingPlayer (frames, encoding, disconnected) {
     this._disconnected = disconnected;
 
     if (this._encoding === undefined) {
-        let frame = this._frames[0];
-        let start = frame.indexOf('{', 1) + 1;
+        const frame = this._frames[0];
+        const start = frame.indexOf('{', 1) + 1;
         if (frame.slice(start).startsWith('UkZC')) {
             this._encoding = 'base64';
         } else {
@@ -108,7 +108,7 @@ RecordingPlayer.prototype = {
     _queueNextPacket: function () {
         if (!this._running) { return; }
 
-        var frame = this._frames[this._frame_index];
+        let frame = this._frames[this._frame_index];
 
         // skip send frames
         while (this._frame_index < this._frame_length && frame.charAt(0) === "}") {
@@ -129,8 +129,8 @@ RecordingPlayer.prototype = {
         }
 
         if (this._realtime) {
-            let foffset = frame.slice(1, frame.indexOf('{', 1));
-            let toffset = (new Date()).getTime() - this._start_time;
+            const foffset = frame.slice(1, frame.indexOf('{', 1));
+            const toffset = (new Date()).getTime() - this._start_time;
             let delay = foffset - toffset;
             if (delay < 1) delay = 1;
 
@@ -143,8 +143,8 @@ RecordingPlayer.prototype = {
     _doPacket: function () {
         // Avoid having excessive queue buildup in non-realtime mode
         if (this._trafficManagement && this._rfb._flushing) {
-            let player = this;
-            let orig = this._rfb._display.onflush;
+            const player = this;
+            const orig = this._rfb._display.onflush;
             this._rfb._display.onflush = function () {
                 player._rfb._display.onflush = orig;
                 player._rfb._onFlush();
@@ -154,8 +154,8 @@ RecordingPlayer.prototype = {
         }
 
         const frame = this._frames[this._frame_index];
-        var start = frame.indexOf('{', 1) + 1;
-        var u8;
+        let start = frame.indexOf('{', 1) + 1;
+        let u8;
         if (this._encoding === 'base64') {
             u8 = Base64.decode(frame.slice(start));
             start = 0;
@@ -174,7 +174,7 @@ RecordingPlayer.prototype = {
 
     _finish() {
         if (this._rfb._display.pending()) {
-            var player = this;
+            const player = this;
             this._rfb._display.onflush = function () {
                 if (player._rfb._flushing) {
                     player._rfb._onFlush();
diff --git a/tests/test.base64.js b/tests/test.base64.js
index b87fdd2..0ff4bfe 100644
--- a/tests/test.base64.js
+++ b/tests/test.base64.js
@@ -1,28 +1,28 @@
-var expect = chai.expect;
+const expect = chai.expect;
 
 import Base64 from '../core/base64.js';
 
 describe('Base64 Tools', function() {
     "use strict";
 
-    var BIN_ARR = new Array(256);
-    for (var i = 0; i < 256; i++) {
+    const BIN_ARR = new Array(256);
+    for (let i = 0; i < 256; i++) {
         BIN_ARR[i] = i;
     }
 
-    var B64_STR = "AAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGxwdHh8gISIjJCUmJygpKissLS4vMDEyMzQ1Njc4OTo7PD0+P0BBQkNERUZHSElKS0xNTk9QUVJTVFVWV1hZWltcXV5fYGFiY2RlZmdoaWprbG1ub3BxcnN0dXZ3eHl6e3x9fn+AgYKDhIWGh4iJiouMjY6PkJGSk5SVlpeYmZqbnJ2en6ChoqOkpaanqKmqq6ytrq+wsbKztLW2t7i5uru8vb6/wMHCw8TFxsfIycrLzM3Oz9DR0tPU1dbX2Nna29zd3t/g4eLj5OXm5+jp6uvs7e7v8PHy8/T19vf4+fr7/P3+/w==";
+    const B64_STR = "AAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGxwdHh8gISIjJCUmJygpKissLS4vMDEyMzQ1Njc4OTo7PD0+P0BBQkNERUZHSElKS0xNTk9QUVJTVFVWV1hZWltcXV5fYGFiY2RlZmdoaWprbG1ub3BxcnN0dXZ3eHl6e3x9fn+AgYKDhIWGh4iJiouMjY6PkJGSk5SVlpeYmZqbnJ2en6ChoqOkpaanqKmqq6ytrq+wsbKztLW2t7i5uru8vb6/wMHCw8TFxsfIycrLzM3Oz9DR0tPU1dbX2Nna29zd3t/g4eLj5OXm5+jp6uvs7e7v8PHy8/T19vf4+fr7/P3+/w==";
 
 
     describe('encode', function() {
         it('should encode a binary string into Base64', function() {
-            var encoded = Base64.encode(BIN_ARR);
+            const encoded = Base64.encode(BIN_ARR);
             expect(encoded).to.equal(B64_STR);
         });
     });
 
     describe('decode', function() {
         it('should decode a Base64 string into a normal string', function() {
-            var decoded = Base64.decode(B64_STR);
+            const decoded = Base64.decode(B64_STR);
             expect(decoded).to.deep.equal(BIN_ARR);
         });
 
diff --git a/tests/test.display.js b/tests/test.display.js
index 5a46f18..a7afd06 100644
--- a/tests/test.display.js
+++ b/tests/test.display.js
@@ -1,4 +1,4 @@
-var expect = chai.expect;
+const expect = chai.expect;
 
 import Base64 from '../core/base64.js';
 import Display from '../core/display.js';
@@ -6,37 +6,35 @@ import Display from '../core/display.js';
 import sinon from '../vendor/sinon.js';
 
 describe('Display/Canvas Helper', function () {
-    var checked_data = [
+    const checked_data = new Uint8Array([
         0x00, 0x00, 0xff, 255, 0x00, 0x00, 0xff, 255, 0x00, 0xff, 0x00, 255, 0x00, 0xff, 0x00, 255,
         0x00, 0x00, 0xff, 255, 0x00, 0x00, 0xff, 255, 0x00, 0xff, 0x00, 255, 0x00, 0xff, 0x00, 255,
         0x00, 0xff, 0x00, 255, 0x00, 0xff, 0x00, 255, 0x00, 0x00, 0xff, 255, 0x00, 0x00, 0xff, 255,
         0x00, 0xff, 0x00, 255, 0x00, 0xff, 0x00, 255, 0x00, 0x00, 0xff, 255, 0x00, 0x00, 0xff, 255
-    ];
-    checked_data = new Uint8Array(checked_data);
+    ]);
 
-    var basic_data = [0xff, 0x00, 0x00, 255, 0x00, 0xff, 0x00, 255, 0x00, 0x00, 0xff, 255, 0xff, 0xff, 0xff, 255];
-    basic_data = new Uint8Array(basic_data);
+    const basic_data = new Uint8Array([0xff, 0x00, 0x00, 255, 0x00, 0xff, 0x00, 255, 0x00, 0x00, 0xff, 255, 0xff, 0xff, 0xff, 255]);
 
     function make_image_canvas (input_data) {
-        var canvas = document.createElement('canvas');
+        const canvas = document.createElement('canvas');
         canvas.width = 4;
         canvas.height = 4;
-        var ctx = canvas.getContext('2d');
-        var data = ctx.createImageData(4, 4);
-        for (var i = 0; i < checked_data.length; i++) { data.data[i] = input_data[i]; }
+        const ctx = canvas.getContext('2d');
+        const data = ctx.createImageData(4, 4);
+        for (let i = 0; i < checked_data.length; i++) { data.data[i] = input_data[i]; }
         ctx.putImageData(data, 0, 0);
         return canvas;
     }
 
     function make_image_png (input_data) {
-        var canvas = make_image_canvas(input_data);
-        var url = canvas.toDataURL();
-        var data = url.split(",")[1];
+        const canvas = make_image_canvas(input_data);
+        const url = canvas.toDataURL();
+        const data = url.split(",")[1];
         return Base64.decode(data);
     }
 
     describe('viewport handling', function () {
-        var display;
+        let display;
         beforeEach(function () {
             display = new Display(document.createElement('canvas'));
             display.clipViewport = true;
@@ -51,10 +49,9 @@ describe('Display/Canvas Helper', function () {
             display.drawImage(make_image_canvas(basic_data), 1, 1);
             display.flip();
 
-            var expected = new Uint8Array(16);
-            var i;
-            for (i = 0; i < 8; i++) { expected[i] = basic_data[i]; }
-            for (i = 8; i < 16; i++) { expected[i] = 0; }
+            const expected = new Uint8Array(16);
+            for (let i = 0; i < 8; i++) { expected[i] = basic_data[i]; }
+            for (let i = 8; i < 16; i++) { expected[i] = 0; }
             expect(display).to.have.displayed(expected);
         });
 
@@ -119,7 +116,7 @@ describe('Display/Canvas Helper', function () {
     });
 
     describe('resizing', function () {
-        var display;
+        let display;
         beforeEach(function () {
             display = new Display(document.createElement('canvas'));
             display.clipViewport = false;
@@ -136,8 +133,8 @@ describe('Display/Canvas Helper', function () {
             display.fillRect(0, 0, 4, 4, [0, 0, 0xff]);
             display.resize(2, 2);
             display.flip();
-            var expected = [];
-            for (var i = 0; i < 4 * 2*2; i += 4) {
+            const expected = [];
+            for (let i = 0; i < 4 * 2*2; i += 4) {
                 expected[i] = 0xff;
                 expected[i+1] = expected[i+2] = 0;
                 expected[i+3] = 0xff;
@@ -179,8 +176,8 @@ describe('Display/Canvas Helper', function () {
     });
 
     describe('rescaling', function () {
-        var display;
-        var canvas;
+        let display;
+        let canvas;
 
         beforeEach(function () {
             canvas = document.createElement('canvas');
@@ -220,8 +217,8 @@ describe('Display/Canvas Helper', function () {
     });
 
     describe('autoscaling', function () {
-        var display;
-        var canvas;
+        let display;
+        let canvas;
 
         beforeEach(function () {
             canvas = document.createElement('canvas');
@@ -268,7 +265,7 @@ describe('Display/Canvas Helper', function () {
 
         // TODO(directxman12): improve the tests for each of the drawing functions to cover more than just the
         //                     basic cases
-        var display;
+        let display;
         beforeEach(function () {
             display = new Display(document.createElement('canvas'));
             display.resize(4, 4);
@@ -279,8 +276,8 @@ describe('Display/Canvas Helper', function () {
             display._logo = null;
             display.clear();
             display.resize(4, 4);
-            var empty = [];
-            for (var i = 0; i < 4 * display._fb_width * display._fb_height; i++) { empty[i] = 0; }
+            const empty = [];
+            for (let i = 0; i < 4 * display._fb_width * display._fb_height; i++) { empty[i] = 0; }
             expect(display).to.have.displayed(new Uint8Array(empty));
         });
 
@@ -300,8 +297,8 @@ describe('Display/Canvas Helper', function () {
             display.fillRect(0, 0, 4, 4, [0, 0, 0xff]);
             display.flip();
             display.fillRect(0, 0, 4, 4, [0, 0xff, 0]);
-            var expected = [];
-            for (var i = 0; i < 4 * display._fb_width * display._fb_height; i += 4) {
+            const expected = [];
+            for (let i = 0; i < 4 * display._fb_width * display._fb_height; i += 4) {
                 expected[i] = 0xff;
                 expected[i+1] = expected[i+2] = 0;
                 expected[i+3] = 0xff;
@@ -346,7 +343,7 @@ describe('Display/Canvas Helper', function () {
 
         // We have a special cache for 16x16 tiles that we need to test
         it('should support drawing a 16x16 tile', function () {
-            let large_checked_data = new Uint8Array(16*16*4);
+            const large_checked_data = new Uint8Array(16*16*4);
             display.resize(16, 16);
 
             for (let y = 0;y < 16;y++) {
@@ -371,8 +368,8 @@ describe('Display/Canvas Helper', function () {
         });
 
         it('should support drawing BGRX blit images with true color via #blitImage', function () {
-            var data = [];
-            for (var i = 0; i < 16; i++) {
+            const data = [];
+            for (let i = 0; i < 16; i++) {
                 data[i * 4] = checked_data[i * 4 + 2];
                 data[i * 4 + 1] = checked_data[i * 4 + 1];
                 data[i * 4 + 2] = checked_data[i * 4];
@@ -384,8 +381,8 @@ describe('Display/Canvas Helper', function () {
         });
 
         it('should support drawing RGB blit images with true color via #blitRgbImage', function () {
-            var data = [];
-            for (var i = 0; i < 16; i++) {
+            const data = [];
+            for (let i = 0; i < 16; i++) {
                 data[i * 3] = checked_data[i * 4];
                 data[i * 3 + 1] = checked_data[i * 4 + 1];
                 data[i * 3 + 2] = checked_data[i * 4 + 2];
@@ -396,7 +393,7 @@ describe('Display/Canvas Helper', function () {
         });
 
         it('should support drawing an image object via #drawImage', function () {
-            var img = make_image_canvas(checked_data);
+            const img = make_image_canvas(checked_data);
             display.drawImage(img, 0, 0);
             display.flip();
             expect(display).to.have.displayed(checked_data);
@@ -404,7 +401,7 @@ describe('Display/Canvas Helper', function () {
     });
 
     describe('the render queue processor', function () {
-        var display;
+        let display;
         beforeEach(function () {
             display = new Display(document.createElement('canvas'));
             display.resize(4, 4);
@@ -427,7 +424,7 @@ describe('Display/Canvas Helper', function () {
         });
 
         it('should wait until an image is loaded to attempt to draw it and the rest of the queue', function () {
-            var img = { complete: false, addEventListener: sinon.spy() }
+            const img = { complete: false, addEventListener: sinon.spy() }
             display._renderQ = [{ type: 'img', x: 3, y: 4, img: img },
                                 { type: 'fill', x: 1, y: 2, width: 3, height: 4, color: 5 }];
             display.drawImage = sinon.spy();
diff --git a/tests/test.helper.js b/tests/test.helper.js
index 1ab02fb..b04005d 100644
--- a/tests/test.helper.js
+++ b/tests/test.helper.js
@@ -1,4 +1,4 @@
-var expect = chai.expect;
+const expect = chai.expect;
 
 import keysyms from '../core/input/keysymdef.js';
 import * as KeyboardUtil from "../core/input/util.js";
@@ -64,7 +64,7 @@ describe('Helpers', function() {
         });
 
         describe('Fix Meta on macOS', function() {
-            var origNavigator;
+            let origNavigator;
             beforeEach(function () {
                 // window.navigator is a protected read-only property in many
                 // environments, so we need to redefine it whilst running these
@@ -127,7 +127,7 @@ describe('Helpers', function() {
         });
 
         describe('Broken key AltGraph on IE/Edge', function() {
-            var origNavigator;
+            let origNavigator;
             beforeEach(function () {
                 // window.navigator is a protected read-only property in many
                 // environments, so we need to redefine it whilst running these
diff --git a/tests/test.keyboard.js b/tests/test.keyboard.js
index 574f88e..3d409fb 100644
--- a/tests/test.keyboard.js
+++ b/tests/test.keyboard.js
@@ -1,4 +1,4 @@
-var expect = chai.expect;
+const expect = chai.expect;
 
 import sinon from '../vendor/sinon.js';
 
@@ -11,8 +11,8 @@ describe('Key Event Handling', function() {
     // The real KeyboardEvent constructor might not work everywhere we
     // want to run these tests
     function keyevent(typeArg, KeyboardEventInit) {
-        var e = { type: typeArg };
-        for (var key in KeyboardEventInit) {
+        const e = { type: typeArg };
+        for (let key in KeyboardEventInit) {
             e[key] = KeyboardEventInit[key];
         }
         e.stopPropagation = sinon.spy();
@@ -23,7 +23,7 @@ describe('Key Event Handling', function() {
     describe('Decode Keyboard Events', function() {
         it('should decode keydown events', function(done) {
             if (browser.isIE() || browser.isEdge()) this.skip();
-            var kbd = new Keyboard(document);
+            const kbd = new Keyboard(document);
             kbd.onkeyevent = function(keysym, code, down) {
                 expect(keysym).to.be.equal(0x61);
                 expect(code).to.be.equal('KeyA');
@@ -34,8 +34,8 @@ describe('Key Event Handling', function() {
         });
         it('should decode keyup events', function(done) {
             if (browser.isIE() || browser.isEdge()) this.skip();
-            var calls = 0;
-            var kbd = new Keyboard(document);
+            let calls = 0;
+            const kbd = new Keyboard(document);
             kbd.onkeyevent = function(keysym, code, down) {
                 expect(keysym).to.be.equal(0x61);
                 expect(code).to.be.equal('KeyA');
@@ -50,13 +50,13 @@ describe('Key Event Handling', function() {
 
         describe('Legacy keypress Events', function() {
             it('should wait for keypress when needed', function() {
-                var kbd = new Keyboard(document);
+                const kbd = new Keyboard(document);
                 kbd.onkeyevent = sinon.spy();
                 kbd._handleKeyDown(keyevent('keydown', {code: 'KeyA', keyCode: 0x41}));
                 expect(kbd.onkeyevent).to.not.have.been.called;
             });
             it('should decode keypress events', function(done) {
-                var kbd = new Keyboard(document);
+                const kbd = new Keyboard(document);
                 kbd.onkeyevent = function(keysym, code, down) {
                     expect(keysym).to.be.equal(0x61);
                     expect(code).to.be.equal('KeyA');
@@ -67,14 +67,14 @@ describe('Key Event Handling', function() {
                 kbd._handleKeyPress(keyevent('keypress', {code: 'KeyA', charCode: 0x61}));
             });
             it('should ignore keypress with different code', function() {
-                var kbd = new Keyboard(document);
+                const kbd = new Keyboard(document);
                 kbd.onkeyevent = sinon.spy();
                 kbd._handleKeyDown(keyevent('keydown', {code: 'KeyA', keyCode: 0x41}));
                 kbd._handleKeyPress(keyevent('keypress', {code: 'KeyB', charCode: 0x61}));
                 expect(kbd.onkeyevent).to.not.have.been.called;
             });
             it('should handle keypress with missing code', function(done) {
-                var kbd = new Keyboard(document);
+                const kbd = new Keyboard(document);
                 kbd.onkeyevent = function(keysym, code, down) {
                     expect(keysym).to.be.equal(0x61);
                     expect(code).to.be.equal('KeyA');
@@ -85,7 +85,7 @@ describe('Key Event Handling', function() {
                 kbd._handleKeyPress(keyevent('keypress', {charCode: 0x61}));
             });
             it('should guess key if no keypress and numeric key', function(done) {
-                var kbd = new Keyboard(document);
+                const kbd = new Keyboard(document);
                 kbd.onkeyevent = function(keysym, code, down) {
                     expect(keysym).to.be.equal(0x32);
                     expect(code).to.be.equal('Digit2');
@@ -95,7 +95,7 @@ describe('Key Event Handling', function() {
                 kbd._handleKeyDown(keyevent('keydown', {code: 'Digit2', keyCode: 0x32}));
             });
             it('should guess key if no keypress and alpha key', function(done) {
-                var kbd = new Keyboard(document);
+                const kbd = new Keyboard(document);
                 kbd.onkeyevent = function(keysym, code, down) {
                     expect(keysym).to.be.equal(0x61);
                     expect(code).to.be.equal('KeyA');
@@ -105,7 +105,7 @@ describe('Key Event Handling', function() {
                 kbd._handleKeyDown(keyevent('keydown', {code: 'KeyA', keyCode: 0x41, shiftKey: false}));
             });
             it('should guess key if no keypress and alpha key (with shift)', function(done) {
-                var kbd = new Keyboard(document);
+                const kbd = new Keyboard(document);
                 kbd.onkeyevent = function(keysym, code, down) {
                     expect(keysym).to.be.equal(0x41);
                     expect(code).to.be.equal('KeyA');
@@ -115,7 +115,7 @@ describe('Key Event Handling', function() {
                 kbd._handleKeyDown(keyevent('keydown', {code: 'KeyA', keyCode: 0x41, shiftKey: true}));
             });
             it('should not guess key if no keypress and unknown key', function(done) {
-                var kbd = new Keyboard(document);
+                const kbd = new Keyboard(document);
                 kbd.onkeyevent = function(keysym, code, down) {
                     expect(keysym).to.be.equal(0);
                     expect(code).to.be.equal('KeyA');
@@ -131,25 +131,25 @@ describe('Key Event Handling', function() {
                 if (browser.isIE() || browser.isEdge()) this.skip();
             });
             it('should suppress anything with a valid key', function() {
-                var kbd = new Keyboard(document, {});
-                var evt1 = keyevent('keydown', {code: 'KeyA', key: 'a'});
+                const kbd = new Keyboard(document, {});
+                const evt1 = keyevent('keydown', {code: 'KeyA', key: 'a'});
                 kbd._handleKeyDown(evt1);
                 expect(evt1.preventDefault).to.have.been.called;
-                var evt2 = keyevent('keyup', {code: 'KeyA', key: 'a'});
+                const evt2 = keyevent('keyup', {code: 'KeyA', key: 'a'});
                 kbd._handleKeyUp(evt2);
                 expect(evt2.preventDefault).to.have.been.called;
             });
             it('should not suppress keys without key', function() {
-                var kbd = new Keyboard(document, {});
-                var evt = keyevent('keydown', {code: 'KeyA', keyCode: 0x41});
+                const kbd = new Keyboard(document, {});
+                const evt = keyevent('keydown', {code: 'KeyA', keyCode: 0x41});
                 kbd._handleKeyDown(evt);
                 expect(evt.preventDefault).to.not.have.been.called;
             });
             it('should suppress the following keypress event', function() {
-                var kbd = new Keyboard(document, {});
-                var evt1 = keyevent('keydown', {code: 'KeyA', keyCode: 0x41});
+                const kbd = new Keyboard(document, {});
+                const evt1 = keyevent('keydown', {code: 'KeyA', keyCode: 0x41});
                 kbd._handleKeyDown(evt1);
-                var evt2 = keyevent('keypress', {code: 'KeyA', charCode: 0x41});
+                const evt2 = keyevent('keypress', {code: 'KeyA', charCode: 0x41});
                 kbd._handleKeyPress(evt2);
                 expect(evt2.preventDefault).to.have.been.called;
             });
@@ -159,8 +159,8 @@ describe('Key Event Handling', function() {
     describe('Fake keyup', function() {
         it('should fake keyup events for virtual keyboards', function(done) {
             if (browser.isIE() || browser.isEdge()) this.skip();
-            var count = 0;
-            var kbd = new Keyboard(document);
+            let count = 0;
+            const kbd = new Keyboard(document);
             kbd.onkeyevent = function(keysym, code, down) {
                 switch (count++) {
                     case 0:
@@ -179,7 +179,7 @@ describe('Key Event Handling', function() {
         });
 
         describe('iOS', function() {
-            var origNavigator;
+            let origNavigator;
             beforeEach(function () {
                 // window.navigator is a protected read-only property in many
                 // environments, so we need to redefine it whilst running these
@@ -206,8 +206,8 @@ describe('Key Event Handling', function() {
 
             it('should fake keyup events on iOS', function(done) {
                 if (browser.isIE() || browser.isEdge()) this.skip();
-                var count = 0;
-                var kbd = new Keyboard(document);
+                let count = 0;
+                const kbd = new Keyboard(document);
                 kbd.onkeyevent = function(keysym, code, down) {
                     switch (count++) {
                         case 0:
@@ -232,7 +232,7 @@ describe('Key Event Handling', function() {
             if (browser.isIE() || browser.isEdge()) this.skip();
         });
         it('should send release using the same keysym as the press', function(done) {
-            var kbd = new Keyboard(document);
+            const kbd = new Keyboard(document);
             kbd.onkeyevent = function(keysym, code, down) {
                 expect(keysym).to.be.equal(0x61);
                 expect(code).to.be.equal('KeyA');
@@ -244,8 +244,8 @@ describe('Key Event Handling', function() {
             kbd._handleKeyUp(keyevent('keyup', {code: 'KeyA', key: 'b'}));
         });
         it('should send the same keysym for multiple presses', function() {
-            var count = 0;
-            var kbd = new Keyboard(document);
+            let count = 0;
+            const kbd = new Keyboard(document);
             kbd.onkeyevent = function(keysym, code, down) {
                 expect(keysym).to.be.equal(0x61);
                 expect(code).to.be.equal('KeyA');
@@ -257,7 +257,7 @@ describe('Key Event Handling', function() {
             expect(count).to.be.equal(2);
         });
         it('should do nothing on keyup events if no keys are down', function() {
-            var kbd = new Keyboard(document);
+            const kbd = new Keyboard(document);
             kbd.onkeyevent = sinon.spy();
             kbd._handleKeyUp(keyevent('keyup', {code: 'KeyA', key: 'a'}));
             expect(kbd.onkeyevent).to.not.have.been.called;
@@ -265,7 +265,7 @@ describe('Key Event Handling', function() {
 
         describe('Legacy Events', function() {
             it('should track keys using keyCode if no code', function(done) {
-                var kbd = new Keyboard(document);
+                const kbd = new Keyboard(document);
                 kbd.onkeyevent = function(keysym, code, down) {
                     expect(keysym).to.be.equal(0x61);
                     expect(code).to.be.equal('Platform65');
@@ -277,7 +277,7 @@ describe('Key Event Handling', function() {
                 kbd._handleKeyUp(keyevent('keyup', {keyCode: 65, key: 'b'}));
             });
             it('should ignore compositing code', function() {
-                var kbd = new Keyboard(document);
+                const kbd = new Keyboard(document);
                 kbd.onkeyevent = function(keysym, code, down) {
                     expect(keysym).to.be.equal(0x61);
                     expect(code).to.be.equal('Unidentified');
@@ -285,7 +285,7 @@ describe('Key Event Handling', function() {
                 kbd._handleKeyDown(keyevent('keydown', {keyCode: 229, key: 'a'}));
             });
             it('should track keys using keyIdentifier if no code', function(done) {
-                var kbd = new Keyboard(document);
+                const kbd = new Keyboard(document);
                 kbd.onkeyevent = function(keysym, code, down) {
                     expect(keysym).to.be.equal(0x61);
                     expect(code).to.be.equal('Platform65');
@@ -300,7 +300,7 @@ describe('Key Event Handling', function() {
     });
 
     describe('Shuffle modifiers on macOS', function() {
-        var origNavigator;
+        let origNavigator;
         beforeEach(function () {
             // window.navigator is a protected read-only property in many
             // environments, so we need to redefine it whilst running these
@@ -326,8 +326,8 @@ describe('Key Event Handling', function() {
         });
 
         it('should change Alt to AltGraph', function() {
-            var count = 0;
-            var kbd = new Keyboard(document);
+            let count = 0;
+            const kbd = new Keyboard(document);
             kbd.onkeyevent = function(keysym, code, down) {
                 switch (count++) {
                     case 0:
@@ -345,7 +345,7 @@ describe('Key Event Handling', function() {
             expect(count).to.be.equal(2);
         });
         it('should change left Super to Alt', function(done) {
-            var kbd = new Keyboard(document);
+            const kbd = new Keyboard(document);
             kbd.onkeyevent = function(keysym, code, down) {
                 expect(keysym).to.be.equal(0xFFE9);
                 expect(code).to.be.equal('MetaLeft');
@@ -354,7 +354,7 @@ describe('Key Event Handling', function() {
             kbd._handleKeyDown(keyevent('keydown', {code: 'MetaLeft', key: 'Meta', location: 1}));
         });
         it('should change right Super to left Super', function(done) {
-            var kbd = new Keyboard(document);
+            const kbd = new Keyboard(document);
             kbd.onkeyevent = function(keysym, code, down) {
                 expect(keysym).to.be.equal(0xFFEB);
                 expect(code).to.be.equal('MetaRight');
@@ -365,7 +365,7 @@ describe('Key Event Handling', function() {
     });
 
     describe('Escape AltGraph on Windows', function() {
-        var origNavigator;
+        let origNavigator;
         beforeEach(function () {
             // window.navigator is a protected read-only property in many
             // environments, so we need to redefine it whilst running these
@@ -394,14 +394,14 @@ describe('Key Event Handling', function() {
         });
 
         it('should supress ControlLeft until it knows if it is AltGr', function () {
-            var kbd = new Keyboard(document);
+            const kbd = new Keyboard(document);
             kbd.onkeyevent = sinon.spy();
             kbd._handleKeyDown(keyevent('keydown', {code: 'ControlLeft', key: 'Control', location: 1}));
             expect(kbd.onkeyevent).to.not.have.been.called;
         });
 
         it('should not trigger on repeating ControlLeft', function () {
-            var kbd = new Keyboard(document);
+            const kbd = new Keyboard(document);
             kbd.onkeyevent = sinon.spy();
             kbd._handleKeyDown(keyevent('keydown', {code: 'ControlLeft', key: 'Control', location: 1}));
             kbd._handleKeyDown(keyevent('keydown', {code: 'ControlLeft', key: 'Control', location: 1}));
@@ -411,7 +411,7 @@ describe('Key Event Handling', function() {
         });
 
         it('should not supress ControlRight', function () {
-            var kbd = new Keyboard(document);
+            const kbd = new Keyboard(document);
             kbd.onkeyevent = sinon.spy();
             kbd._handleKeyDown(keyevent('keydown', {code: 'ControlRight', key: 'Control', location: 2}));
             expect(kbd.onkeyevent).to.have.been.calledOnce;
@@ -419,7 +419,7 @@ describe('Key Event Handling', function() {
         });
 
         it('should release ControlLeft after 100 ms', function () {
-            var kbd = new Keyboard(document);
+            const kbd = new Keyboard(document);
             kbd.onkeyevent = sinon.spy();
             kbd._handleKeyDown(keyevent('keydown', {code: 'ControlLeft', key: 'Control', location: 1}));
             expect(kbd.onkeyevent).to.not.have.been.called;
@@ -429,7 +429,7 @@ describe('Key Event Handling', function() {
         });
 
         it('should release ControlLeft on other key press', function () {
-            var kbd = new Keyboard(document);
+            const kbd = new Keyboard(document);
             kbd.onkeyevent = sinon.spy();
             kbd._handleKeyDown(keyevent('keydown', {code: 'ControlLeft', key: 'Control', location: 1}));
             expect(kbd.onkeyevent).to.not.have.been.called;
@@ -445,7 +445,7 @@ describe('Key Event Handling', function() {
         });
 
         it('should release ControlLeft on other key release', function () {
-            var kbd = new Keyboard(document);
+            const kbd = new Keyboard(document);
             kbd.onkeyevent = sinon.spy();
             kbd._handleKeyDown(keyevent('keydown', {code: 'KeyA', key: 'a'}));
             kbd._handleKeyDown(keyevent('keydown', {code: 'ControlLeft', key: 'Control', location: 1}));
@@ -463,7 +463,7 @@ describe('Key Event Handling', function() {
         });
 
         it('should generate AltGraph for quick Ctrl+Alt sequence', function () {
-            var kbd = new Keyboard(document);
+            const kbd = new Keyboard(document);
             kbd.onkeyevent = sinon.spy();
             kbd._handleKeyDown(keyevent('keydown', {code: 'ControlLeft', key: 'Control', location: 1, timeStamp: Date.now()}));
             this.clock.tick(20);
@@ -478,7 +478,7 @@ describe('Key Event Handling', function() {
         });
 
         it('should generate Ctrl, Alt for slow Ctrl+Alt sequence', function () {
-            var kbd = new Keyboard(document);
+            const kbd = new Keyboard(document);
             kbd.onkeyevent = sinon.spy();
             kbd._handleKeyDown(keyevent('keydown', {code: 'ControlLeft', key: 'Control', location: 1, timeStamp: Date.now()}));
             this.clock.tick(60);
@@ -494,7 +494,7 @@ describe('Key Event Handling', function() {
         });
 
         it('should pass through single Alt', function () {
-            var kbd = new Keyboard(document);
+            const kbd = new Keyboard(document);
             kbd.onkeyevent = sinon.spy();
             kbd._handleKeyDown(keyevent('keydown', {code: 'AltRight', key: 'Alt', location: 2}));
             expect(kbd.onkeyevent).to.have.been.calledOnce;
@@ -502,7 +502,7 @@ describe('Key Event Handling', function() {
         });
 
         it('should pass through single AltGr', function () {
-            var kbd = new Keyboard(document);
+            const kbd = new Keyboard(document);
             kbd.onkeyevent = sinon.spy();
             kbd._handleKeyDown(keyevent('keydown', {code: 'AltRight', key: 'AltGraph', location: 2}));
             expect(kbd.onkeyevent).to.have.been.calledOnce;
diff --git a/tests/test.localization.js b/tests/test.localization.js
index adb29db..7ea11d6 100644
--- a/tests/test.localization.js
+++ b/tests/test.localization.js
@@ -1,12 +1,11 @@
-var expect = chai.expect;
-
+const expect = chai.expect;
 import { l10n } from '../app/localization.js';
 
 describe('Localization', function() {
     "use strict";
 
     describe('language selection', function () {
-        var origNavigator;
+        let origNavigator;
         beforeEach(function () {
             // window.navigator is a protected read-only property in many
             // environments, so we need to redefine it whilst running these
diff --git a/tests/test.mouse.js b/tests/test.mouse.js
index d25913c..58200a4 100644
--- a/tests/test.mouse.js
+++ b/tests/test.mouse.js
@@ -1,4 +1,4 @@
-var expect = chai.expect;
+const expect = chai.expect;
 
 import sinon from '../vendor/sinon.js';
 
@@ -14,24 +14,24 @@ describe('Mouse Event Handling', function() {
     // located at coordinates 10x10
     sinon.stub(Element.prototype, 'getBoundingClientRect').returns(
         {left: 10, right: 110, top: 10, bottom: 110, width: 100, height: 100});
-    var target = document.createElement('canvas');
+    const target = document.createElement('canvas');
 
     // The real constructors might not work everywhere we
     // want to run these tests
-    var mouseevent, touchevent;
-    mouseevent = touchevent = function(typeArg, MouseEventInit) {
-        var e = { type: typeArg };
-        for (var key in MouseEventInit) {
+    const mouseevent = function(typeArg, MouseEventInit) {
+        const e = { type: typeArg };
+        for (let key in MouseEventInit) {
             e[key] = MouseEventInit[key];
         }
         e.stopPropagation = sinon.spy();
         e.preventDefault = sinon.spy();
         return e;
     };
+    const touchevent = mouseevent;
 
     describe('Decode Mouse Events', function() {
         it('should decode mousedown events', function(done) {
-            var mouse = new Mouse(target);
+            const mouse = new Mouse(target);
             mouse.onmousebutton = function(x, y, down, bmask) {
                 expect(bmask).to.be.equal(0x01);
                 expect(down).to.be.equal(1);
@@ -40,8 +40,8 @@ describe('Mouse Event Handling', function() {
             mouse._handleMouseDown(mouseevent('mousedown', { button: '0x01' }));
         });
         it('should decode mouseup events', function(done) {
-            var calls = 0;
-            var mouse = new Mouse(target);
+            let calls = 0;
+            const mouse = new Mouse(target);
             mouse.onmousebutton = function(x, y, down, bmask) {
                 expect(bmask).to.be.equal(0x01);
                 if (calls++ === 1) {
@@ -53,7 +53,7 @@ describe('Mouse Event Handling', function() {
             mouse._handleMouseUp(mouseevent('mouseup', { button: '0x01' }));
         });
         it('should decode mousemove events', function(done) {
-            var mouse = new Mouse(target);
+            const mouse = new Mouse(target);
             mouse.onmousemove = function(x, y) {
                 // Note that target relative coordinates are sent
                 expect(x).to.be.equal(40);
@@ -64,8 +64,8 @@ describe('Mouse Event Handling', function() {
                                               { clientX: 50, clientY: 20 }));
         });
         it('should decode mousewheel events', function(done) {
-            var calls = 0;
-            var mouse = new Mouse(target);
+            let calls = 0;
+            const mouse = new Mouse(target);
             mouse.onmousebutton = function(x, y, down, bmask) {
                 calls++;
                 expect(bmask).to.be.equal(1<<6);
@@ -88,8 +88,8 @@ describe('Mouse Event Handling', function() {
         afterEach(function () { this.clock.restore(); });
 
         it('should use same pos for 2nd tap if close enough', function(done) {
-            var calls = 0;
-            var mouse = new Mouse(target);
+            let calls = 0;
+            const mouse = new Mouse(target);
             mouse.onmousebutton = function(x, y, down, bmask) {
                 calls++;
                 if (calls === 1) {
@@ -119,8 +119,8 @@ describe('Mouse Event Handling', function() {
         });
 
         it('should not modify 2nd tap pos if far apart', function(done) {
-            var calls = 0;
-            var mouse = new Mouse(target);
+            let calls = 0;
+            const mouse = new Mouse(target);
             mouse.onmousebutton = function(x, y, down, bmask) {
                 calls++;
                 if (calls === 1) {
@@ -148,8 +148,8 @@ describe('Mouse Event Handling', function() {
         });
 
         it('should not modify 2nd tap pos if not soon enough', function(done) {
-            var calls = 0;
-            var mouse = new Mouse(target);
+            let calls = 0;
+            const mouse = new Mouse(target);
             mouse.onmousebutton = function(x, y, down, bmask) {
                 calls++;
                 if (calls === 1) {
@@ -177,8 +177,8 @@ describe('Mouse Event Handling', function() {
         });
 
         it('should not modify 2nd tap pos if not touch', function(done) {
-            var calls = 0;
-            var mouse = new Mouse(target);
+            let calls = 0;
+            const mouse = new Mouse(target);
             mouse.onmousebutton = function(x, y, down, bmask) {
                 calls++;
                 if (calls === 1) {
@@ -213,7 +213,7 @@ describe('Mouse Event Handling', function() {
         afterEach(function () { this.clock.restore(); });
 
         it('should accumulate wheel events if small enough', function () {
-            var mouse = new Mouse(target);
+            const mouse = new Mouse(target);
             mouse.onmousebutton = sinon.spy();
 
             mouse._handleMouseWheel(mouseevent(
@@ -246,7 +246,7 @@ describe('Mouse Event Handling', function() {
         });
 
         it('should not accumulate large wheel events', function () {
-            var mouse = new Mouse(target);
+            const mouse = new Mouse(target);
             mouse.onmousebutton = sinon.spy();
 
             mouse._handleMouseWheel(mouseevent(
@@ -265,7 +265,7 @@ describe('Mouse Event Handling', function() {
         });
 
         it('should send even small wheel events after a timeout', function () {
-            var mouse = new Mouse(target);
+            const mouse = new Mouse(target);
             mouse.onmousebutton = sinon.spy();
 
             mouse._handleMouseWheel(mouseevent(
@@ -277,7 +277,7 @@ describe('Mouse Event Handling', function() {
         });
 
         it('should account for non-zero deltaMode', function () {
-            var mouse = new Mouse(target);
+            const mouse = new Mouse(target);
             mouse.onmousebutton = sinon.spy();
 
             mouse._handleMouseWheel(mouseevent(
diff --git a/tests/test.rfb.js b/tests/test.rfb.js
index eaf1366..21cfc35 100644
--- a/tests/test.rfb.js
+++ b/tests/test.rfb.js
@@ -1,4 +1,4 @@
-var expect = chai.expect;
+const expect = chai.expect;
 
 import RFB from '../core/rfb.js';
 import Websock from '../core/websock.js';
@@ -13,7 +13,7 @@ import sinon from '../vendor/sinon.js';
 
     function UIEvent ( event, params ) {
         params = params || { bubbles: false, cancelable: false, view: window, detail: undefined };
-        var evt = document.createEvent( 'UIEvent' );
+        const evt = document.createEvent( 'UIEvent' );
         evt.initUIEvent( event, params.bubbles, params.cancelable, params.view, params.detail );
         return evt;
     }
@@ -23,18 +23,18 @@ import sinon from '../vendor/sinon.js';
     window.UIEvent = UIEvent;
 })();
 
-var push8 = function (arr, num) {
+const push8 = function (arr, num) {
     "use strict";
     arr.push(num & 0xFF);
 };
 
-var push16 = function (arr, num) {
+const push16 = function (arr, num) {
     "use strict";
     arr.push((num >> 8) & 0xFF,
               num & 0xFF);
 };
 
-var push32 = function (arr, num) {
+const push32 = function (arr, num) {
     "use strict";
     arr.push((num >> 24) & 0xFF,
               (num >> 16) & 0xFF,
@@ -43,8 +43,8 @@ var push32 = function (arr, num) {
 };
 
 describe('Remote Frame Buffer Protocol Client', function() {
-    var clock;
-    var raf;
+    let clock;
+    let raf;
 
     before(FakeWebSocket.replace);
     after(FakeWebSocket.restore);
@@ -56,9 +56,9 @@ describe('Remote Frame Buffer Protocol Client', function() {
         window.requestAnimationFrame = setTimeout;
         // Use a single set of buffers instead of reallocating to
         // speed up tests
-        var sock = new Websock();
-        var _sQ = new Uint8Array(sock._sQbufferSize);
-        var rQ = new Uint8Array(sock._rQbufferSize);
+        const sock = new Websock();
+        const _sQ = new Uint8Array(sock._sQbufferSize);
+        const rQ = new Uint8Array(sock._rQbufferSize);
 
         Websock.prototype._old_allocate_buffers = Websock.prototype._allocate_buffers;
         Websock.prototype._allocate_buffers = function () {
@@ -74,8 +74,8 @@ describe('Remote Frame Buffer Protocol Client', function() {
         window.requestAnimationFrame = raf;
     });
 
-    var container;
-    var rfbs;
+    let container;
+    let rfbs;
 
     beforeEach(function () {
         // Create a container element for all RFB objects to attach to
@@ -102,7 +102,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
 
     function make_rfb (url, options) {
         url = url || 'wss://host:8675';
-        var rfb = new RFB(container, url, options);
+        const rfb = new RFB(container, url, options);
         clock.tick();
         rfb._sock._websocket._open();
         rfb._rfb_connection_state = 'connected';
@@ -114,14 +114,14 @@ describe('Remote Frame Buffer Protocol Client', function() {
     describe('Connecting/Disconnecting', function () {
         describe('#RFB', function () {
             it('should set the current state to "connecting"', function () {
-                var client = new RFB(document.createElement('div'), 'wss://host:8675');
+                const client = new RFB(document.createElement('div'), 'wss://host:8675');
                 client._rfb_connection_state = '';
                 this.clock.tick();
                 expect(client._rfb_connection_state).to.equal('connecting');
             });
 
             it('should actually connect to the websocket', function () {
-                var client = new RFB(document.createElement('div'), 'ws://HOST:8675/PATH');
+                const client = new RFB(document.createElement('div'), 'ws://HOST:8675/PATH');
                 sinon.spy(client._sock, 'open');
                 this.clock.tick();
                 expect(client._sock.open).to.have.been.calledOnce;
@@ -130,7 +130,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
         });
 
         describe('#disconnect', function () {
-            var client;
+            let client;
             beforeEach(function () {
                 client = make_rfb();
             });
@@ -166,7 +166,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
         });
 
         describe('#sendCredentials', function () {
-            var client;
+            let client;
             beforeEach(function () {
                 client = make_rfb();
                 client._rfb_connection_state = 'connecting';
@@ -187,14 +187,14 @@ describe('Remote Frame Buffer Protocol Client', function() {
     });
 
     describe('Public API Basic Behavior', function () {
-        var client;
+        let client;
         beforeEach(function () {
             client = make_rfb();
         });
 
         describe('#sendCtrlAlDel', function () {
             it('should sent ctrl[down]-alt[down]-del[down] then del[up]-alt[up]-ctrl[up]', function () {
-                var expected = {_sQ: new Uint8Array(48), _sQlen: 0, flush: function () {}};
+                const expected = {_sQ: new Uint8Array(48), _sQlen: 0, flush: function () {}};
                 RFB.messages.keyEvent(expected, 0xFFE3, 1);
                 RFB.messages.keyEvent(expected, 0xFFE9, 1);
                 RFB.messages.keyEvent(expected, 0xFFFF, 1);
@@ -223,14 +223,14 @@ describe('Remote Frame Buffer Protocol Client', function() {
 
         describe('#sendKey', function () {
             it('should send a single key with the given code and state (down = true)', function () {
-                var expected = {_sQ: new Uint8Array(8), _sQlen: 0, flush: function () {}};
+                const expected = {_sQ: new Uint8Array(8), _sQlen: 0, flush: function () {}};
                 RFB.messages.keyEvent(expected, 123, 1);
                 client.sendKey(123, 'Key123', true);
                 expect(client._sock).to.have.sent(expected._sQ);
             });
 
             it('should send both a down and up event if the state is not specified', function () {
-                var expected = {_sQ: new Uint8Array(16), _sQlen: 0, flush: function () {}};
+                const expected = {_sQ: new Uint8Array(16), _sQlen: 0, flush: function () {}};
                 RFB.messages.keyEvent(expected, 123, 1);
                 RFB.messages.keyEvent(expected, 123, 0);
                 client.sendKey(123, 'Key123');
@@ -253,7 +253,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
 
             it('should send QEMU extended events if supported', function () {
                 client._qemuExtKeyEventSupported = true;
-                var expected = {_sQ: new Uint8Array(12), _sQlen: 0, flush: function () {}};
+                const expected = {_sQ: new Uint8Array(12), _sQlen: 0, flush: function () {}};
                 RFB.messages.QEMUExtendedKeyEvent(expected, 0x20, true, 0x0039);
                 client.sendKey(0x20, 'Space', true);
                 expect(client._sock).to.have.sent(expected._sQ);
@@ -261,7 +261,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
 
             it('should not send QEMU extended events if unknown key code', function () {
                 client._qemuExtKeyEventSupported = true;
-                var expected = {_sQ: new Uint8Array(8), _sQlen: 0, flush: function () {}};
+                const expected = {_sQ: new Uint8Array(8), _sQlen: 0, flush: function () {}};
                 RFB.messages.keyEvent(expected, 123, 1);
                 client.sendKey(123, 'FooBar', true);
                 expect(client._sock).to.have.sent(expected._sQ);
@@ -286,7 +286,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
 
         describe('#clipboardPasteFrom', function () {
             it('should send the given text in a paste event', function () {
-                var expected = {_sQ: new Uint8Array(11), _sQlen: 0,
+                const expected = {_sQ: new Uint8Array(11), _sQlen: 0,
                                 _sQbufferSize: 11, flush: function () {}};
                 RFB.messages.clientCutText(expected, 'abc');
                 client.clipboardPasteFrom('abc');
@@ -340,7 +340,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
     });
 
     describe('Clipping', function () {
-        var client;
+        let client;
         beforeEach(function () {
             client = make_rfb();
             container.style.width = '70px';
@@ -349,7 +349,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
         });
 
         it('should update display clip state when changing the property', function () {
-            var spy = sinon.spy(client._display, "clipViewport", ["set"]);
+            const spy = sinon.spy(client._display, "clipViewport", ["set"]);
 
             client.clipViewport = false;
             expect(spy.set).to.have.been.calledOnce;
@@ -366,7 +366,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
 
             container.style.width = '40px';
             container.style.height = '50px';
-            var event = new UIEvent('resize');
+            const event = new UIEvent('resize');
             window.dispatchEvent(event);
             clock.tick();
 
@@ -376,7 +376,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
 
         it('should update the viewport when the remote session resizes', function () {
             // Simple ExtendedDesktopSize FBU message
-            var incoming = [ 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x00,
+            const incoming = [ 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x00,
                              0x00, 0xff, 0x00, 0xff, 0xff, 0xff, 0xfe, 0xcc,
                              0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
                              0x00, 0x00, 0x00, 0x00, 0x00, 0xff, 0x00, 0xff,
@@ -398,7 +398,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
 
             container.style.width = '40px';
             container.style.height = '50px';
-            var event = new UIEvent('resize');
+            const event = new UIEvent('resize');
             window.dispatchEvent(event);
             clock.tick();
 
@@ -411,7 +411,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
 
             container.style.width = '40px';
             container.style.height = '50px';
-            var event = new UIEvent('resize');
+            const event = new UIEvent('resize');
             window.dispatchEvent(event);
             clock.tick();
 
@@ -513,7 +513,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
     });
 
     describe('Scaling', function () {
-        var client;
+        let client;
         beforeEach(function () {
             client = make_rfb();
             container.style.width = '70px';
@@ -522,7 +522,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
         });
 
         it('should update display scale factor when changing the property', function () {
-            var spy = sinon.spy(client._display, "scale", ["set"]);
+            const spy = sinon.spy(client._display, "scale", ["set"]);
             sinon.spy(client._display, "autoscale");
 
             client.scaleViewport = false;
@@ -538,7 +538,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
         it('should update the clipping setting when changing the property', function () {
             client.clipViewport = true;
 
-            var spy = sinon.spy(client._display, "clipViewport", ["set"]);
+            const spy = sinon.spy(client._display, "clipViewport", ["set"]);
 
             client.scaleViewport = false;
             expect(spy.set).to.have.been.calledOnce;
@@ -556,7 +556,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
 
             container.style.width = '40px';
             container.style.height = '50px';
-            var event = new UIEvent('resize');
+            const event = new UIEvent('resize');
             window.dispatchEvent(event);
             clock.tick();
 
@@ -566,7 +566,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
 
         it('should update the scaling when the remote session resizes', function () {
             // Simple ExtendedDesktopSize FBU message
-            var incoming = [ 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x00,
+            const incoming = [ 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x00,
                              0x00, 0xff, 0x00, 0xff, 0xff, 0xff, 0xfe, 0xcc,
                              0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
                              0x00, 0x00, 0x00, 0x00, 0x00, 0xff, 0x00, 0xff,
@@ -587,7 +587,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
 
             container.style.width = '40px';
             container.style.height = '50px';
-            var event = new UIEvent('resize');
+            const event = new UIEvent('resize');
             window.dispatchEvent(event);
             clock.tick();
 
@@ -596,7 +596,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
     });
 
     describe('Remote resize', function () {
-        var client;
+        let client;
         beforeEach(function () {
             client = make_rfb();
             client._supportsSetDesktopSize = true;
@@ -619,7 +619,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
 
         it('should request a resize when initially connecting', function () {
             // Simple ExtendedDesktopSize FBU message
-            var incoming = [ 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x00,
+            const incoming = [ 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x00,
                              0x00, 0x04, 0x00, 0x04, 0xff, 0xff, 0xfe, 0xcc,
                              0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
                              0x00, 0x00, 0x00, 0x00, 0x00, 0x04, 0x00, 0x04,
@@ -646,7 +646,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
         it('should request a resize when the container resizes', function () {
             container.style.width = '40px';
             container.style.height = '50px';
-            var event = new UIEvent('resize');
+            const event = new UIEvent('resize');
             window.dispatchEvent(event);
             clock.tick(1000);
 
@@ -657,7 +657,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
         it('should not resize until the container size is stable', function () {
             container.style.width = '20px';
             container.style.height = '30px';
-            var event1 = new UIEvent('resize');
+            const event1 = new UIEvent('resize');
             window.dispatchEvent(event1);
             clock.tick(400);
 
@@ -665,7 +665,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
 
             container.style.width = '40px';
             container.style.height = '50px';
-            var event2 = new UIEvent('resize');
+            const event2 = new UIEvent('resize');
             window.dispatchEvent(event2);
             clock.tick(400);
 
@@ -682,7 +682,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
 
             container.style.width = '40px';
             container.style.height = '50px';
-            var event = new UIEvent('resize');
+            const event = new UIEvent('resize');
             window.dispatchEvent(event);
             clock.tick(1000);
 
@@ -694,7 +694,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
 
             container.style.width = '40px';
             container.style.height = '50px';
-            var event = new UIEvent('resize');
+            const event = new UIEvent('resize');
             window.dispatchEvent(event);
             clock.tick(1000);
 
@@ -706,7 +706,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
 
             container.style.width = '40px';
             container.style.height = '50px';
-            var event = new UIEvent('resize');
+            const event = new UIEvent('resize');
             window.dispatchEvent(event);
             clock.tick(1000);
 
@@ -715,7 +715,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
 
         it('should not try to override a server resize', function () {
             // Simple ExtendedDesktopSize FBU message
-            var incoming = [ 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x00,
+            const incoming = [ 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x00,
                              0x00, 0x04, 0x00, 0x04, 0xff, 0xff, 0xfe, 0xcc,
                              0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
                              0x00, 0x00, 0x00, 0x00, 0x00, 0x04, 0x00, 0x04,
@@ -729,13 +729,13 @@ describe('Remote Frame Buffer Protocol Client', function() {
 
     describe('Misc Internals', function () {
         describe('#_updateConnectionState', function () {
-            var client;
+            let client;
             beforeEach(function () {
                 client = make_rfb();
             });
 
             it('should clear the disconnect timer if the state is not "disconnecting"', function () {
-                var spy = sinon.spy();
+                const spy = sinon.spy();
                 client._disconnTimer = setTimeout(spy, 50);
                 client._rfb_connection_state = 'connecting';
                 client._updateConnectionState('connected');
@@ -758,7 +758,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
             });
 
             it('should ignore state changes to the same state', function () {
-                var connectSpy = sinon.spy();
+                const connectSpy = sinon.spy();
                 client.addEventListener("connect", connectSpy);
 
                 expect(client._rfb_connection_state).to.equal('connected');
@@ -767,7 +767,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
 
                 client.disconnect();
 
-                var disconnectSpy = sinon.spy();
+                const disconnectSpy = sinon.spy();
                 client.addEventListener("disconnect", disconnectSpy);
 
                 expect(client._rfb_connection_state).to.equal('disconnected');
@@ -776,7 +776,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
             });
 
             it('should ignore illegal state changes', function () {
-                var spy = sinon.spy();
+                const spy = sinon.spy();
                 client.addEventListener("disconnect", spy);
                 client._updateConnectionState('disconnected');
                 expect(client._rfb_connection_state).to.not.equal('disconnected');
@@ -785,7 +785,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
         });
 
         describe('#_fail', function () {
-            var client;
+            let client;
             beforeEach(function () {
                 client = make_rfb();
             });
@@ -813,7 +813,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
 
             it('should result in disconnect event with clean set to false', function () {
                 client._rfb_connection_state = 'connected';
-                var spy = sinon.spy();
+                const spy = sinon.spy();
                 client.addEventListener("disconnect", spy);
                 client._fail();
                 this.clock.tick(2000);
@@ -827,7 +827,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
     describe('Connection States', function () {
         describe('connecting', function () {
             it('should open the websocket connection', function () {
-                var client = new RFB(document.createElement('div'),
+                const client = new RFB(document.createElement('div'),
                                      'ws://HOST:8675/PATH');
                 sinon.spy(client._sock, 'open');
                 this.clock.tick();
@@ -836,13 +836,13 @@ describe('Remote Frame Buffer Protocol Client', function() {
         });
 
         describe('connected', function () {
-            var client;
+            let client;
             beforeEach(function () {
                 client = make_rfb();
             });
 
             it('should result in a connect event if state becomes connected', function () {
-                var spy = sinon.spy();
+                const spy = sinon.spy();
                 client.addEventListener("connect", spy);
                 client._rfb_connection_state = 'connecting';
                 client._updateConnectionState('connected');
@@ -850,7 +850,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
             });
 
             it('should not result in a connect event if the state is not "connected"', function () {
-                var spy = sinon.spy();
+                const spy = sinon.spy();
                 client.addEventListener("connect", spy);
                 client._sock._websocket.open = function () {};  // explicitly don't call onopen
                 client._updateConnectionState('connecting');
@@ -859,7 +859,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
         });
 
         describe('disconnecting', function () {
-            var client;
+            let client;
             beforeEach(function () {
                 client = make_rfb();
             });
@@ -889,7 +889,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
             });
 
             it('should not result in a disconnect event', function () {
-                var spy = sinon.spy();
+                const spy = sinon.spy();
                 client.addEventListener("disconnect", spy);
                 client._sock._websocket.close = function () {};  // explicitly don't call onclose
                 client._updateConnectionState('disconnecting');
@@ -898,13 +898,13 @@ describe('Remote Frame Buffer Protocol Client', function() {
         });
 
         describe('disconnected', function () {
-            var client;
+            let client;
             beforeEach(function () {
                 client = new RFB(document.createElement('div'), 'ws://HOST:8675/PATH');
             });
 
             it('should result in a disconnect event if state becomes "disconnected"', function () {
-                var spy = sinon.spy();
+                const spy = sinon.spy();
                 client.addEventListener("disconnect", spy);
                 client._rfb_connection_state = 'disconnecting';
                 client._updateConnectionState('disconnected');
@@ -913,7 +913,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
             });
 
             it('should result in a disconnect event without msg when no reason given', function () {
-                var spy = sinon.spy();
+                const spy = sinon.spy();
                 client.addEventListener("disconnect", spy);
                 client._rfb_connection_state = 'disconnecting';
                 client._rfb_disconnect_reason = "";
@@ -925,7 +925,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
     });
 
     describe('Protocol Initialization States', function () {
-        var client;
+        let client;
         beforeEach(function () {
             client = make_rfb();
             client._rfb_connection_state = 'connecting';
@@ -933,8 +933,8 @@ describe('Remote Frame Buffer Protocol Client', function() {
 
         describe('ProtocolVersion', function () {
             function send_ver (ver, client) {
-                var arr = new Uint8Array(12);
-                for (var i = 0; i < ver.length; i++) {
+                const arr = new Uint8Array(12);
+                for (let i = 0; i < ver.length; i++) {
                     arr[i+4] = ver.charCodeAt(i);
                 }
                 arr[0] = 'R'; arr[1] = 'F'; arr[2] = 'B'; arr[3] = ' ';
@@ -993,9 +993,9 @@ describe('Remote Frame Buffer Protocol Client', function() {
             it('should send back the interpreted version', function () {
                 send_ver('004.000', client);
 
-                var expected_str = 'RFB 003.008\n';
-                var expected = [];
-                for (var i = 0; i < expected_str.length; i++) {
+                const expected_str = 'RFB 003.008\n';
+                const expected = [];
+                for (let i = 0; i < expected_str.length; i++) {
                     expected[i] = expected_str.charCodeAt(i);
                 }
 
@@ -1017,7 +1017,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
                     send_ver('000.000', client);
                     expect(client._rfb_version).to.equal(0);
 
-                    var sent_data = client._sock._websocket._get_sent_data();
+                    const sent_data = client._sock._websocket._get_sent_data();
                     expect(new Uint8Array(sent_data.buffer, 0, 9)).to.array.equal(new Uint8Array([73, 68, 58, 49, 50, 51, 52, 53, 0]));
                     expect(sent_data).to.have.length(250);
                 });
@@ -1037,8 +1037,8 @@ describe('Remote Frame Buffer Protocol Client', function() {
 
             it('should simply receive the auth scheme when for versions < 3.7', function () {
                 client._rfb_version = 3.6;
-                var auth_scheme_raw = [1, 2, 3, 4];
-                var auth_scheme = (auth_scheme_raw[0] << 24) + (auth_scheme_raw[1] << 16) +
+                const auth_scheme_raw = [1, 2, 3, 4];
+                const auth_scheme = (auth_scheme_raw[0] << 24) + (auth_scheme_raw[1] << 16) +
                                   (auth_scheme_raw[2] << 8) + auth_scheme_raw[3];
                 client._sock._websocket._receive_data(auth_scheme_raw);
                 expect(client._rfb_auth_scheme).to.equal(auth_scheme);
@@ -1046,7 +1046,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
 
             it('should prefer no authentication is possible', function () {
                 client._rfb_version = 3.7;
-                var auth_schemes = [2, 1, 3];
+                const auth_schemes = [2, 1, 3];
                 client._sock._websocket._receive_data(auth_schemes);
                 expect(client._rfb_auth_scheme).to.equal(1);
                 expect(client._sock).to.have.sent(new Uint8Array([1, 1]));
@@ -1054,7 +1054,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
 
             it('should choose for the most prefered scheme possible for versions >= 3.7', function () {
                 client._rfb_version = 3.7;
-                var auth_schemes = [2, 22, 16];
+                const auth_schemes = [2, 22, 16];
                 client._sock._websocket._receive_data(auth_schemes);
                 expect(client._rfb_auth_scheme).to.equal(22);
                 expect(client._sock).to.have.sent(new Uint8Array([22]));
@@ -1063,14 +1063,14 @@ describe('Remote Frame Buffer Protocol Client', function() {
             it('should fail if there are no supported schemes for versions >= 3.7', function () {
                 sinon.spy(client, "_fail");
                 client._rfb_version = 3.7;
-                var auth_schemes = [1, 32];
+                const auth_schemes = [1, 32];
                 client._sock._websocket._receive_data(auth_schemes);
                 expect(client._fail).to.have.been.calledOnce;
             });
 
             it('should fail with the appropriate message if no types are sent for versions >= 3.7', function () {
                 client._rfb_version = 3.7;
-                var failure_data = [0, 0, 0, 0, 6, 119, 104, 111, 111, 112, 115];
+                const failure_data = [0, 0, 0, 0, 6, 119, 104, 111, 111, 112, 115];
                 sinon.spy(client, '_fail');
                 client._sock._websocket._receive_data(failure_data);
 
@@ -1081,7 +1081,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
 
             it('should transition to the Authentication state and continue on successful negotiation', function () {
                 client._rfb_version = 3.7;
-                var auth_schemes = [1, 1];
+                const auth_schemes = [1, 1];
                 client._negotiate_authentication = sinon.spy();
                 client._sock._websocket._receive_data(auth_schemes);
                 expect(client._rfb_init_state).to.equal('Authentication');
@@ -1100,11 +1100,11 @@ describe('Remote Frame Buffer Protocol Client', function() {
 
             it('should fail on auth scheme 0 (pre 3.7) with the given message', function () {
                 client._rfb_version = 3.6;
-                var err_msg = "Whoopsies";
-                var data = [0, 0, 0, 0];
-                var err_len = err_msg.length;
+                const err_msg = "Whoopsies";
+                const data = [0, 0, 0, 0];
+                const err_len = err_msg.length;
                 push32(data, err_len);
-                for (var i = 0; i < err_len; i++) {
+                for (let i = 0; i < err_len; i++) {
                     data.push(err_msg.charCodeAt(i));
                 }
 
@@ -1140,12 +1140,12 @@ describe('Remote Frame Buffer Protocol Client', function() {
                 });
 
                 it('should fire the credentialsrequired event if missing a password', function () {
-                    var spy = sinon.spy();
+                    const spy = sinon.spy();
                     client.addEventListener("credentialsrequired", spy);
                     send_security(2, client);
 
-                    var challenge = [];
-                    for (var i = 0; i < 16; i++) { challenge[i] = i; }
+                    const challenge = [];
+                    for (let i = 0; i < 16; i++) { challenge[i] = i; }
                     client._sock._websocket._receive_data(new Uint8Array(challenge));
 
                     expect(client._rfb_credentials).to.be.empty;
@@ -1158,11 +1158,11 @@ describe('Remote Frame Buffer Protocol Client', function() {
                     send_security(2, client);
                     client._sock._websocket._get_sent_data(); // skip the choice of auth reply
 
-                    var challenge = [];
-                    for (var i = 0; i < 16; i++) { challenge[i] = i; }
+                    const challenge = [];
+                    for (let i = 0; i < 16; i++) { challenge[i] = i; }
                     client._sock._websocket._receive_data(new Uint8Array(challenge));
 
-                    var des_pass = RFB.genDES('passwd', challenge);
+                    const des_pass = RFB.genDES('passwd', challenge);
                     expect(client._sock).to.have.sent(new Uint8Array(des_pass));
                 });
 
@@ -1170,8 +1170,8 @@ describe('Remote Frame Buffer Protocol Client', function() {
                     client._rfb_credentials = { password: 'passwd' };
                     send_security(2, client);
 
-                    var challenge = [];
-                    for (var i = 0; i < 16; i++) { challenge[i] = i; }
+                    const challenge = [];
+                    for (let i = 0; i < 16; i++) { challenge[i] = i; }
                     client._sock._websocket._receive_data(new Uint8Array(challenge));
 
                     expect(client._rfb_init_state).to.equal('SecurityResult');
@@ -1194,7 +1194,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
                 });
 
                 it('should fire the credentialsrequired event if all credentials are missing', function() {
-                    var spy = sinon.spy();
+                    const spy = sinon.spy();
                     client.addEventListener("credentialsrequired", spy);
                     client._rfb_credentials = {};
                     send_security(22, client);
@@ -1205,7 +1205,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
                 });
 
                 it('should fire the credentialsrequired event if some credentials are missing', function() {
-                    var spy = sinon.spy();
+                    const spy = sinon.spy();
                     client.addEventListener("credentialsrequired", spy);
                     client._rfb_credentials = { username: 'user',
                                                 target: 'target' };
@@ -1223,8 +1223,8 @@ describe('Remote Frame Buffer Protocol Client', function() {
 
                     send_security(22, client);
 
-                    var expected = [22, 4, 6]; // auth selection, len user, len target
-                    for (var i = 0; i < 10; i++) { expected[i+3] = 'usertarget'.charCodeAt(i); }
+                    const expected = [22, 4, 6]; // auth selection, len user, len target
+                    for (let i = 0; i < 10; i++) { expected[i+3] = 'usertarget'.charCodeAt(i); }
 
                     expect(client._sock).to.have.sent(new Uint8Array(expected));
                 });
@@ -1239,17 +1239,16 @@ describe('Remote Frame Buffer Protocol Client', function() {
                 });
 
                 function send_num_str_pairs(pairs, client) {
-                    var pairs_len = pairs.length;
-                    var data = [];
+                    const pairs_len = pairs.length;
+                    const data = [];
                     push32(data, pairs_len);
 
-                    for (var i = 0; i < pairs_len; i++) {
+                    for (let i = 0; i < pairs_len; i++) {
                         push32(data, pairs[i][0]);
-                        var j;
-                        for (j = 0; j < 4; j++) {
+                        for (let j = 0; j < 4; j++) {
                             data.push(pairs[i][1].charCodeAt(j));
                         }
-                        for (j = 0; j < 8; j++) {
+                        for (let j = 0; j < 8; j++) {
                             data.push(pairs[i][2].charCodeAt(j));
                         }
                     }
@@ -1328,7 +1327,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
             it('should fail on an error code of 1 with the given message for versions >= 3.8', function () {
                 client._rfb_version = 3.8;
                 sinon.spy(client, '_fail');
-                var failure_data = [0, 0, 0, 1, 0, 0, 0, 6, 119, 104, 111, 111, 112, 115];
+                const failure_data = [0, 0, 0, 1, 0, 0, 0, 6, 119, 104, 111, 111, 112, 115];
                 client._sock._websocket._receive_data(new Uint8Array(failure_data));
                 expect(client._fail).to.have.been.calledWith(
                     'Security negotiation failed on security result (reason: whoops)');
@@ -1343,7 +1342,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
             });
 
             it('should result in securityfailure event when receiving a non zero status', function () {
-                var spy = sinon.spy();
+                const spy = sinon.spy();
                 client.addEventListener("securityfailure", spy);
                 client._sock._websocket._receive_data(new Uint8Array([0, 0, 0, 2]));
                 expect(spy).to.have.been.calledOnce;
@@ -1352,9 +1351,9 @@ describe('Remote Frame Buffer Protocol Client', function() {
 
             it('should include reason when provided in securityfailure event', function () {
                 client._rfb_version = 3.8;
-                var spy = sinon.spy();
+                const spy = sinon.spy();
                 client.addEventListener("securityfailure", spy);
-                var failure_data = [0, 0, 0, 1, 0, 0, 0, 12, 115, 117, 99, 104,
+                const failure_data = [0, 0, 0, 1, 0, 0, 0, 12, 115, 117, 99, 104,
                                     32, 102, 97, 105, 108, 117, 114, 101];
                 client._sock._websocket._receive_data(new Uint8Array(failure_data));
                 expect(spy.args[0][0].detail.status).to.equal(1);
@@ -1363,9 +1362,9 @@ describe('Remote Frame Buffer Protocol Client', function() {
 
             it('should not include reason when length is zero in securityfailure event', function () {
                 client._rfb_version = 3.9;
-                var spy = sinon.spy();
+                const spy = sinon.spy();
                 client.addEventListener("securityfailure", spy);
-                var failure_data = [0, 0, 0, 1, 0, 0, 0, 0];
+                const failure_data = [0, 0, 0, 1, 0, 0, 0, 0];
                 client._sock._websocket._receive_data(new Uint8Array(failure_data));
                 expect(spy.args[0][0].detail.status).to.equal(1);
                 expect('reason' in spy.args[0][0].detail).to.be.false;
@@ -1373,7 +1372,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
 
             it('should not include reason in securityfailure event for version < 3.8', function () {
                 client._rfb_version = 3.6;
-                var spy = sinon.spy();
+                const spy = sinon.spy();
                 client.addEventListener("securityfailure", spy);
                 client._sock._websocket._receive_data(new Uint8Array([0, 0, 0, 2]));
                 expect(spy.args[0][0].detail.status).to.equal(2);
@@ -1383,7 +1382,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
 
         describe('ClientInitialisation', function () {
             it('should transition to the ServerInitialisation state', function () {
-                var client = make_rfb();
+                const client = make_rfb();
                 client._rfb_connection_state = 'connecting';
                 client._rfb_init_state = 'SecurityResult';
                 client._sock._websocket._receive_data(new Uint8Array([0, 0, 0, 0]));
@@ -1391,7 +1390,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
             });
 
             it('should send 1 if we are in shared mode', function () {
-                var client = make_rfb('wss://host:8675', { shared: true });
+                const client = make_rfb('wss://host:8675', { shared: true });
                 client._rfb_connection_state = 'connecting';
                 client._rfb_init_state = 'SecurityResult';
                 client._sock._websocket._receive_data(new Uint8Array([0, 0, 0, 0]));
@@ -1399,7 +1398,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
             });
 
             it('should send 0 if we are not in shared mode', function () {
-                var client = make_rfb('wss://host:8675', { shared: false });
+                const client = make_rfb('wss://host:8675', { shared: false });
                 client._rfb_connection_state = 'connecting';
                 client._rfb_init_state = 'SecurityResult';
                 client._sock._websocket._receive_data(new Uint8Array([0, 0, 0, 0]));
@@ -1413,13 +1412,13 @@ describe('Remote Frame Buffer Protocol Client', function() {
             });
 
             function send_server_init(opts, client) {
-                var full_opts = { width: 10, height: 12, bpp: 24, depth: 24, big_endian: 0,
+                const full_opts = { width: 10, height: 12, bpp: 24, depth: 24, big_endian: 0,
                                   true_color: 1, red_max: 255, green_max: 255, blue_max: 255,
                                   red_shift: 16, green_shift: 8, blue_shift: 0, name: 'a name' };
-                for (var opt in opts) {
+                for (let opt in opts) {
                     full_opts[opt] = opts[opt];
                 }
-                var data = [];
+                const data = [];
 
                 push16(data, full_opts.width);
                 push16(data, full_opts.height);
@@ -1443,9 +1442,9 @@ describe('Remote Frame Buffer Protocol Client', function() {
 
                 client._sock._websocket._receive_data(new Uint8Array(data));
 
-                var name_data = [];
+                const name_data = [];
                 push32(name_data, full_opts.name.length);
-                for (var i = 0; i < full_opts.name.length; i++) {
+                for (let i = 0; i < full_opts.name.length; i++) {
                     name_data.push(full_opts.name.charCodeAt(i));
                 }
                 client._sock._websocket._receive_data(new Uint8Array(name_data));
@@ -1460,7 +1459,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
             // NB(sross): we just warn, not fail, for endian-ness and shifts, so we don't test them
 
             it('should set the framebuffer name and call the callback', function () {
-                var spy = sinon.spy();
+                const spy = sinon.spy();
                 client.addEventListener("desktopname", spy);
                 send_server_init({ name: 'some name' }, client);
 
@@ -1475,12 +1474,12 @@ describe('Remote Frame Buffer Protocol Client', function() {
                 client._rfb_tightvnc = true;
                 send_server_init({}, client);
 
-                var tight_data = [];
+                const tight_data = [];
                 push16(tight_data, 1);
                 push16(tight_data, 2);
                 push16(tight_data, 3);
                 push16(tight_data, 0);
-                for (var i = 0; i < 16 + 32 + 48; i++) {
+                for (let i = 0; i < 16 + 32 + 48; i++) {
                     tight_data.push(i);
                 }
                 client._sock._websocket._receive_data(tight_data);
@@ -1554,7 +1553,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
     });
 
     describe('Protocol Message Processing After Completing Initialization', function () {
-        var client;
+        let client;
 
         beforeEach(function () {
             client = make_rfb();
@@ -1564,21 +1563,21 @@ describe('Remote Frame Buffer Protocol Client', function() {
         });
 
         describe('Framebuffer Update Handling', function () {
-            var target_data_arr = [
+            const target_data_arr = [
                 0xff, 0x00, 0x00, 255, 0x00, 0xff, 0x00, 255, 0x00, 0x00, 0xff, 255, 0x00, 0x00, 0xff, 255,
                 0x00, 0xff, 0x00, 255, 0xff, 0x00, 0x00, 255, 0x00, 0x00, 0xff, 255, 0x00, 0x00, 0xff, 255,
                 0xee, 0x00, 0xff, 255, 0x00, 0xee, 0xff, 255, 0xaa, 0xee, 0xff, 255, 0xab, 0xee, 0xff, 255,
                 0xee, 0x00, 0xff, 255, 0x00, 0xee, 0xff, 255, 0xaa, 0xee, 0xff, 255, 0xab, 0xee, 0xff, 255
             ];
-            var target_data;
+            let target_data;
 
-            var target_data_check_arr = [
+            const target_data_check_arr = [
                 0x00, 0x00, 0xff, 255, 0x00, 0x00, 0xff, 255, 0x00, 0xff, 0x00, 255, 0x00, 0xff, 0x00, 255,
                 0x00, 0x00, 0xff, 255, 0x00, 0x00, 0xff, 255, 0x00, 0xff, 0x00, 255, 0x00, 0xff, 0x00, 255,
                 0x00, 0xff, 0x00, 255, 0x00, 0xff, 0x00, 255, 0x00, 0x00, 0xff, 255, 0x00, 0x00, 0xff, 255,
                 0x00, 0xff, 0x00, 255, 0x00, 0xff, 0x00, 255, 0x00, 0x00, 0xff, 255, 0x00, 0x00, 0xff, 255
             ];
-            var target_data_check;
+            let target_data_check;
 
             before(function () {
                 // NB(directxman12): PhantomJS 1.x doesn't implement Uint8ClampedArray
@@ -1587,7 +1586,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
             });
 
             function send_fbu_msg (rect_info, rect_data, client, rect_cnt) {
-                var data = [];
+                let data = [];
 
                 if (!rect_cnt || rect_cnt > -1) {
                     // header
@@ -1596,7 +1595,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
                     push16(data, rect_cnt || rect_data.length);
                 }
 
-                for (var i = 0; i < rect_data.length; i++) {
+                for (let i = 0; i < rect_data.length; i++) {
                     if (rect_info[i]) {
                         push16(data, rect_info[i].x);
                         push16(data, rect_info[i].y);
@@ -1611,7 +1610,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
             }
 
             it('should send an update request if there is sufficient data', function () {
-                var expected_msg = {_sQ: new Uint8Array(10), _sQlen: 0, flush: function() {}};
+                const expected_msg = {_sQ: new Uint8Array(10), _sQlen: 0, flush: function() {}};
                 RFB.messages.fbUpdateRequest(expected_msg, true, 0, 0, 640, 20);
 
                 client._framebufferUpdate = function () { return true; };
@@ -1626,7 +1625,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
             });
 
             it('should resume receiving an update if we previously did not have enough data', function () {
-                var expected_msg = {_sQ: new Uint8Array(10), _sQlen: 0, flush: function() {}};
+                const expected_msg = {_sQ: new Uint8Array(10), _sQlen: 0, flush: function() {}};
                 RFB.messages.fbUpdateRequest(expected_msg, true, 0, 0, 640, 20);
 
                 // just enough to set FBU.rects
@@ -1649,7 +1648,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
 
             it('should fail on an unsupported encoding', function () {
                 sinon.spy(client, "_fail");
-                var rect_info = { x: 8, y: 11, width: 27, height: 32, encoding: 234 };
+                const rect_info = { x: 8, y: 11, width: 27, height: 32, encoding: 234 };
                 send_fbu_msg([rect_info], [[]], client);
                 expect(client._fail).to.have.been.calledOnce;
             });
@@ -1661,10 +1660,10 @@ describe('Remote Frame Buffer Protocol Client', function() {
                 client._display.resize(4, 4);
                 client._display.blitRgbxImage(0, 0, 4, 2, new Uint8Array(target_data_check_arr.slice(0, 32)), 0);
 
-                var info = [{ x: 0, y: 2, width: 2, height: 2, encoding: 0x01},
+                const info = [{ x: 0, y: 2, width: 2, height: 2, encoding: 0x01},
                             { x: 2, y: 2, width: 2, height: 2, encoding: 0x01}];
                 // data says [{ old_x: 2, old_y: 0 }, { old_x: 0, old_y: 0 }]
-                var rects = [[0, 2, 0, 0], [0, 0, 0, 0]];
+                const rects = [[0, 2, 0, 0], [0, 0, 0, 0]];
                 send_fbu_msg([info[0]], [rects[0]], client, 2);
                 send_fbu_msg([info[1]], [rects[1]], client, -1);
                 expect(client._display).to.have.displayed(target_data_check);
@@ -1680,12 +1679,12 @@ describe('Remote Frame Buffer Protocol Client', function() {
                 });
 
                 it('should handle the RAW encoding', function () {
-                    var info = [{ x: 0, y: 0, width: 2, height: 2, encoding: 0x00 },
+                    const info = [{ x: 0, y: 0, width: 2, height: 2, encoding: 0x00 },
                                 { x: 2, y: 0, width: 2, height: 2, encoding: 0x00 },
                                 { x: 0, y: 2, width: 4, height: 1, encoding: 0x00 },
                                 { x: 0, y: 3, width: 4, height: 1, encoding: 0x00 }];
                     // data is in bgrx
-                    var rects = [
+                    const rects = [
                         [0x00, 0x00, 0xff, 0, 0x00, 0xff, 0x00, 0, 0x00, 0xff, 0x00, 0, 0x00, 0x00, 0xff, 0],
                         [0xff, 0x00, 0x00, 0, 0xff, 0x00, 0x00, 0, 0xff, 0x00, 0x00, 0, 0xff, 0x00, 0x00, 0],
                         [0xff, 0x00, 0xee, 0, 0xff, 0xee, 0x00, 0, 0xff, 0xee, 0xaa, 0, 0xff, 0xee, 0xab, 0],
@@ -1695,11 +1694,11 @@ describe('Remote Frame Buffer Protocol Client', function() {
                 });
 
                 it('should handle the RAW encoding in low colour mode', function () {
-                    var info = [{ x: 0, y: 0, width: 2, height: 2, encoding: 0x00 },
+                    const info = [{ x: 0, y: 0, width: 2, height: 2, encoding: 0x00 },
                                 { x: 2, y: 0, width: 2, height: 2, encoding: 0x00 },
                                 { x: 0, y: 2, width: 4, height: 1, encoding: 0x00 },
                                 { x: 0, y: 3, width: 4, height: 1, encoding: 0x00 }];
-                    var rects = [
+                    const rects = [
                         [0x03, 0x03, 0x03, 0x03],
                         [0x0c, 0x0c, 0x0c, 0x0c],
                         [0x0c, 0x0c, 0x03, 0x03],
@@ -1713,10 +1712,10 @@ describe('Remote Frame Buffer Protocol Client', function() {
                     // seed some initial data to copy
                     client._display.blitRgbxImage(0, 0, 4, 2, new Uint8Array(target_data_check_arr.slice(0, 32)), 0);
 
-                    var info = [{ x: 0, y: 2, width: 2, height: 2, encoding: 0x01},
+                    const info = [{ x: 0, y: 2, width: 2, height: 2, encoding: 0x01},
                                 { x: 2, y: 2, width: 2, height: 2, encoding: 0x01}];
                     // data says [{ old_x: 0, old_y: 0 }, { old_x: 0, old_y: 0 }]
-                    var rects = [[0, 2, 0, 0], [0, 0, 0, 0]];
+                    const rects = [[0, 2, 0, 0], [0, 0, 0, 0]];
                     send_fbu_msg(info, rects, client);
                     expect(client._display).to.have.displayed(target_data_check);
                 });
@@ -1725,8 +1724,8 @@ describe('Remote Frame Buffer Protocol Client', function() {
                 // TODO(directxman12): test rre_chunk_sz (related to above about subrects)?
 
                 it('should handle the RRE encoding', function () {
-                    var info = [{ x: 0, y: 0, width: 4, height: 4, encoding: 0x02 }];
-                    var rect = [];
+                    const info = [{ x: 0, y: 0, width: 4, height: 4, encoding: 0x02 }];
+                    const rect = [];
                     push32(rect, 2); // 2 subrects
                     push32(rect, 0xff00ff); // becomes 00ff00ff --> #00FF00 bg color
                     rect.push(0xff); // becomes ff0000ff --> #0000FF color
@@ -1752,8 +1751,8 @@ describe('Remote Frame Buffer Protocol Client', function() {
 
                 describe('the HEXTILE encoding handler', function () {
                     it('should handle a tile with fg, bg specified, normal subrects', function () {
-                        var info = [{ x: 0, y: 0, width: 4, height: 4, encoding: 0x05 }];
-                        var rect = [];
+                        const info = [{ x: 0, y: 0, width: 4, height: 4, encoding: 0x05 }];
+                        const rect = [];
                         rect.push(0x02 | 0x04 | 0x08); // bg spec, fg spec, anysubrects
                         push32(rect, 0xff00ff); // becomes 00ff00ff --> #00FF00 bg color
                         rect.push(0xff); // becomes ff0000ff --> #0000FF fg color
@@ -1770,10 +1769,10 @@ describe('Remote Frame Buffer Protocol Client', function() {
                     });
 
                     it('should handle a raw tile', function () {
-                        var info = [{ x: 0, y: 0, width: 4, height: 4, encoding: 0x05 }];
-                        var rect = [];
+                        const info = [{ x: 0, y: 0, width: 4, height: 4, encoding: 0x05 }];
+                        const rect = [];
                         rect.push(0x01); // raw
-                        for (var i = 0; i < target_data.length; i += 4) {
+                        for (let i = 0; i < target_data.length; i += 4) {
                             rect.push(target_data[i + 2]);
                             rect.push(target_data[i + 1]);
                             rect.push(target_data[i]);
@@ -1784,14 +1783,14 @@ describe('Remote Frame Buffer Protocol Client', function() {
                     });
 
                     it('should handle a tile with only bg specified (solid bg)', function () {
-                        var info = [{ x: 0, y: 0, width: 4, height: 4, encoding: 0x05 }];
-                        var rect = [];
+                        const info = [{ x: 0, y: 0, width: 4, height: 4, encoding: 0x05 }];
+                        const rect = [];
                         rect.push(0x02);
                         push32(rect, 0xff00ff); // becomes 00ff00ff --> #00FF00 bg color
                         send_fbu_msg(info, [rect], client);
 
-                        var expected = [];
-                        for (var i = 0; i < 16; i++) { push32(expected, 0xff00ff); }
+                        const expected = [];
+                        for (let i = 0; i < 16; i++) { push32(expected, 0xff00ff); }
                         expect(client._display).to.have.displayed(new Uint8Array(expected));
                     });
 
@@ -1800,9 +1799,9 @@ describe('Remote Frame Buffer Protocol Client', function() {
                         client._fb_width = 8;
                         client._display.resize(8, 4);
 
-                        var info = [{ x: 0, y: 0, width: 32, height: 4, encoding: 0x05 }];
+                        const info = [{ x: 0, y: 0, width: 32, height: 4, encoding: 0x05 }];
 
-                        var rect = [];
+                        const rect = [];
 
                         // send a bg frame
                         rect.push(0x02);
@@ -1813,16 +1812,15 @@ describe('Remote Frame Buffer Protocol Client', function() {
 
                         send_fbu_msg(info, [rect], client);
 
-                        var expected = [];
-                        var i;
-                        for (i = 0; i < 16; i++) { push32(expected, 0xff00ff); }     // rect 1: solid
-                        for (i = 0; i < 16; i++) { push32(expected, 0xff00ff); }    // rect 2: same bkground color
+                        const expected = [];
+                        for (let i = 0; i < 16; i++) { push32(expected, 0xff00ff); }     // rect 1: solid
+                        for (let i = 0; i < 16; i++) { push32(expected, 0xff00ff); }    // rect 2: same bkground color
                         expect(client._display).to.have.displayed(new Uint8Array(expected));
                     });
 
                     it('should handle a tile with bg and coloured subrects', function () {
-                        var info = [{ x: 0, y: 0, width: 4, height: 4, encoding: 0x05 }];
-                        var rect = [];
+                        const info = [{ x: 0, y: 0, width: 4, height: 4, encoding: 0x05 }];
+                        const rect = [];
                         rect.push(0x02 | 0x08 | 0x10); // bg spec, anysubrects, colouredsubrects
                         push32(rect, 0xff00ff); // becomes 00ff00ff --> #00FF00 bg color
                         rect.push(2); // 2 subrects
@@ -1847,8 +1845,8 @@ describe('Remote Frame Buffer Protocol Client', function() {
                         client._fb_height = 17;
                         client._display.resize(4, 17);
 
-                        var info = [{ x: 0, y: 0, width: 4, height: 17, encoding: 0x05}];
-                        var rect = [];
+                        const info = [{ x: 0, y: 0, width: 4, height: 17, encoding: 0x05}];
+                        const rect = [];
                         rect.push(0x02 | 0x04 | 0x08); // bg spec, fg spec, anysubrects
                         push32(rect, 0xff00ff); // becomes 00ff00ff --> #00FF00 bg color
                         rect.push(0xff); // becomes ff0000ff --> #0000FF fg color
@@ -1856,8 +1854,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
                         rect.push(0x00);
                         rect.push(0xff);
                         rect.push(8); // 8 subrects
-                        var i;
-                        for (i = 0; i < 4; i++) {
+                        for (let i = 0; i < 4; i++) {
                             rect.push((0 << 4) | (i * 4)); // x: 0, y: i*4
                             rect.push(1 | (1 << 4)); // width: 2, height: 2
                             rect.push((2 << 4) | (i * 4 + 2)); // x: 2, y: i * 4 + 2
@@ -1869,16 +1866,16 @@ describe('Remote Frame Buffer Protocol Client', function() {
                         rect.push(1 | (1 << 4)); // width: 2, height: 2
                         send_fbu_msg(info, [rect], client);
 
-                        var expected = [];
-                        for (i = 0; i < 4; i++) { expected = expected.concat(target_data_check_arr); }
+                        let expected = [];
+                        for (let i = 0; i < 4; i++) { expected = expected.concat(target_data_check_arr); }
                         expected = expected.concat(target_data_check_arr.slice(0, 16));
                         expect(client._display).to.have.displayed(new Uint8Array(expected));
                     });
 
                     it('should fail on an invalid subencoding', function () {
                         sinon.spy(client,"_fail");
-                        var info = [{ x: 0, y: 0, width: 4, height: 4, encoding: 0x05 }];
-                        var rects = [[45]];  // an invalid subencoding
+                        const info = [{ x: 0, y: 0, width: 4, height: 4, encoding: 0x05 }];
+                        const rects = [[45]];  // an invalid subencoding
                         send_fbu_msg(info, rects, client);
                         expect(client._fail).to.have.been.calledOnce;
                     });
@@ -1913,11 +1910,11 @@ describe('Remote Frame Buffer Protocol Client', function() {
                     });
 
                     function make_screen_data (nr_of_screens) {
-                        var data = [];
+                        const data = [];
                         push8(data, nr_of_screens);   // number-of-screens
                         push8(data, 0);               // padding
                         push16(data, 0);              // padding
-                        for (var i=0; i<nr_of_screens; i += 1) {
+                        for (let i=0; i<nr_of_screens; i += 1) {
                             push32(data, 0);  // id
                             push16(data, 0);  // x-position
                             push16(data, 0);  // y-position
@@ -1929,8 +1926,8 @@ describe('Remote Frame Buffer Protocol Client', function() {
                     }
 
                     it('should handle a resize requested by this client', function () {
-                        var reason_for_change = 1; // requested by this client
-                        var status_code       = 0; // No error
+                        const reason_for_change = 1; // requested by this client
+                        const status_code       = 0; // No error
 
                         send_fbu_msg([{ x: reason_for_change, y: status_code,
                                         width: 20, height: 50, encoding: -308 }],
@@ -1944,8 +1941,8 @@ describe('Remote Frame Buffer Protocol Client', function() {
                     });
 
                     it('should handle a resize requested by another client', function () {
-                        var reason_for_change = 2; // requested by another client
-                        var status_code       = 0; // No error
+                        const reason_for_change = 2; // requested by another client
+                        const status_code       = 0; // No error
 
                         send_fbu_msg([{ x: reason_for_change, y: status_code,
                                         width: 20, height: 50, encoding: -308 }],
@@ -1959,8 +1956,8 @@ describe('Remote Frame Buffer Protocol Client', function() {
                     });
 
                     it('should be able to recieve requests which contain data for multiple screens', function () {
-                        var reason_for_change = 2; // requested by another client
-                        var status_code       = 0; // No error
+                        const reason_for_change = 2; // requested by another client
+                        const status_code       = 0; // No error
 
                         send_fbu_msg([{ x: reason_for_change, y: status_code,
                                         width: 60, height: 50, encoding: -308 }],
@@ -1974,8 +1971,8 @@ describe('Remote Frame Buffer Protocol Client', function() {
                     });
 
                     it('should not handle a failed request', function () {
-                        var reason_for_change = 1; // requested by this client
-                        var status_code       = 1; // Resize is administratively prohibited
+                        const reason_for_change = 1; // requested by this client
+                        const status_code       = 1; // Resize is administratively prohibited
 
                         send_fbu_msg([{ x: reason_for_change, y: status_code,
                                         width: 20, height: 50, encoding: -308 }],
@@ -2001,7 +1998,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
 
         describe('XVP Message Handling', function () {
             it('should set the XVP version and fire the callback with the version on XVP_INIT', function () {
-                var spy = sinon.spy();
+                const spy = sinon.spy();
                 client.addEventListener("capabilities", spy);
                 client._sock._websocket._receive_data(new Uint8Array([250, 0, 10, 1]));
                 expect(client._rfb_xvp_ver).to.equal(10);
@@ -2018,11 +2015,11 @@ describe('Remote Frame Buffer Protocol Client', function() {
         });
 
         it('should fire the clipboard callback with the retrieved text on ServerCutText', function () {
-            var expected_str = 'cheese!';
-            var data = [3, 0, 0, 0];
+            const expected_str = 'cheese!';
+            const data = [3, 0, 0, 0];
             push32(data, expected_str.length);
-            for (var i = 0; i < expected_str.length; i++) { data.push(expected_str.charCodeAt(i)); }
-            var spy = sinon.spy();
+            for (let i = 0; i < expected_str.length; i++) { data.push(expected_str.charCodeAt(i)); }
+            const spy = sinon.spy();
             client.addEventListener("clipboard", spy);
 
             client._sock._websocket._receive_data(new Uint8Array(data));
@@ -2031,17 +2028,17 @@ describe('Remote Frame Buffer Protocol Client', function() {
         });
 
         it('should fire the bell callback on Bell', function () {
-            var spy = sinon.spy();
+            const spy = sinon.spy();
             client.addEventListener("bell", spy);
             client._sock._websocket._receive_data(new Uint8Array([2]));
             expect(spy).to.have.been.calledOnce;
         });
 
         it('should respond correctly to ServerFence', function () {
-            var expected_msg = {_sQ: new Uint8Array(16), _sQlen: 0, flush: function() {}};
-            var incoming_msg = {_sQ: new Uint8Array(16), _sQlen: 0, flush: function() {}};
+            const expected_msg = {_sQ: new Uint8Array(16), _sQlen: 0, flush: function() {}};
+            const incoming_msg = {_sQ: new Uint8Array(16), _sQlen: 0, flush: function() {}};
 
-            var payload = "foo\x00ab9";
+            const payload = "foo\x00ab9";
 
             // ClientFence and ServerFence are identical in structure
             RFB.messages.clientFence(expected_msg, (1<<0) | (1<<1), payload);
@@ -2063,7 +2060,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
         });
 
         it('should enable continuous updates on first EndOfContinousUpdates', function () {
-            var expected_msg = {_sQ: new Uint8Array(10), _sQlen: 0, flush: function() {}};
+            const expected_msg = {_sQ: new Uint8Array(10), _sQlen: 0, flush: function() {}};
 
             RFB.messages.enableContinuousUpdates(expected_msg, true, 0, 0, 640, 20);
 
@@ -2085,7 +2082,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
         });
 
         it('should update continuous updates on resize', function () {
-            var expected_msg = {_sQ: new Uint8Array(10), _sQlen: 0, flush: function() {}};
+            const expected_msg = {_sQ: new Uint8Array(10), _sQlen: 0, flush: function() {}};
             RFB.messages.enableContinuousUpdates(expected_msg, true, 0, 0, 90, 700);
 
             client._resize(450, 160);
@@ -2107,7 +2104,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
     });
 
     describe('Asynchronous Events', function () {
-        var client;
+        let client;
         beforeEach(function () {
             client = make_rfb();
         });
@@ -2129,14 +2126,14 @@ describe('Remote Frame Buffer Protocol Client', function() {
 
             it('should send a pointer event on mouse button presses', function () {
                 client._handleMouseButton(10, 12, 1, 0x001);
-                var pointer_msg = {_sQ: new Uint8Array(6), _sQlen: 0, flush: function () {}};
+                const pointer_msg = {_sQ: new Uint8Array(6), _sQlen: 0, flush: function () {}};
                 RFB.messages.pointerEvent(pointer_msg, 10, 12, 0x001);
                 expect(client._sock).to.have.sent(pointer_msg._sQ);
             });
 
             it('should send a mask of 1 on mousedown', function () {
                 client._handleMouseButton(10, 12, 1, 0x001);
-                var pointer_msg = {_sQ: new Uint8Array(6), _sQlen: 0, flush: function () {}};
+                const pointer_msg = {_sQ: new Uint8Array(6), _sQlen: 0, flush: function () {}};
                 RFB.messages.pointerEvent(pointer_msg, 10, 12, 0x001);
                 expect(client._sock).to.have.sent(pointer_msg._sQ);
             });
@@ -2144,14 +2141,14 @@ describe('Remote Frame Buffer Protocol Client', function() {
             it('should send a mask of 0 on mouseup', function () {
                 client._mouse_buttonMask = 0x001;
                 client._handleMouseButton(10, 12, 0, 0x001);
-                var pointer_msg = {_sQ: new Uint8Array(6), _sQlen: 0, flush: function () {}};
+                const pointer_msg = {_sQ: new Uint8Array(6), _sQlen: 0, flush: function () {}};
                 RFB.messages.pointerEvent(pointer_msg, 10, 12, 0x000);
                 expect(client._sock).to.have.sent(pointer_msg._sQ);
             });
 
             it('should send a pointer event on mouse movement', function () {
                 client._handleMouseMove(10, 12);
-                var pointer_msg = {_sQ: new Uint8Array(6), _sQlen: 0, flush: function () {}};
+                const pointer_msg = {_sQ: new Uint8Array(6), _sQlen: 0, flush: function () {}};
                 RFB.messages.pointerEvent(pointer_msg, 10, 12, 0x000);
                 expect(client._sock).to.have.sent(pointer_msg._sQ);
             });
@@ -2159,7 +2156,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
             it('should set the button mask so that future mouse movements use it', function () {
                 client._handleMouseButton(10, 12, 1, 0x010);
                 client._handleMouseMove(13, 9);
-                var pointer_msg = {_sQ: new Uint8Array(12), _sQlen: 0, flush: function () {}};
+                const pointer_msg = {_sQ: new Uint8Array(12), _sQlen: 0, flush: function () {}};
                 RFB.messages.pointerEvent(pointer_msg, 10, 12, 0x010);
                 RFB.messages.pointerEvent(pointer_msg, 13, 9, 0x010);
                 expect(client._sock).to.have.sent(pointer_msg._sQ);
@@ -2169,7 +2166,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
         describe('Keyboard Event Handlers', function () {
             it('should send a key message on a key press', function () {
                 client._handleKeyEvent(0x41, 'KeyA', true);
-                var key_msg = {_sQ: new Uint8Array(8), _sQlen: 0, flush: function () {}};
+                const key_msg = {_sQ: new Uint8Array(8), _sQlen: 0, flush: function () {}};
                 RFB.messages.keyEvent(key_msg, 0x41, 1);
                 expect(client._sock).to.have.sent(key_msg._sQ);
             });
@@ -2205,7 +2202,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
             });
 
             it('should process all normal messages directly', function () {
-                var spy = sinon.spy();
+                const spy = sinon.spy();
                 client.addEventListener("bell", spy);
                 client._sock._websocket._receive_data(new Uint8Array([0x02, 0x02]));
                 expect(spy).to.have.been.calledTwice;
@@ -2228,7 +2225,7 @@ describe('Remote Frame Buffer Protocol Client', function() {
 
             // close events
             it('should transition to "disconnected" from "disconnecting" on a close event', function () {
-                var real = client._sock._websocket.close;
+                const real = client._sock._websocket.close;
                 client._sock._websocket.close = function () {};
                 client.disconnect();
                 expect(client._rfb_connection_state).to.equal('disconnecting');
diff --git a/tests/test.util.js b/tests/test.util.js
index 09c4ef0..c73e4ff 100644
--- a/tests/test.util.js
+++ b/tests/test.util.js
@@ -1,5 +1,5 @@
 /* eslint-disable no-console */
-var expect = chai.expect;
+const expect = chai.expect;
 
 import * as Log from '../core/util/logging.js';
 
diff --git a/tests/test.websock.js b/tests/test.websock.js
index 1e978ec..02ec270 100644
--- a/tests/test.websock.js
+++ b/tests/test.websock.js
@@ -1,4 +1,4 @@
-var expect = chai.expect;
+const expect = chai.expect;
 
 import Websock from '../core/websock.js';
 import FakeWebSocket from './fake.websocket.js';
@@ -9,8 +9,8 @@ describe('Websock', function() {
     "use strict";
 
     describe('Queue methods', function () {
-        var sock;
-        var RQ_TEMPLATE = new Uint8Array([0, 1, 2, 3, 4, 5, 6, 7]);
+        let sock;
+        const RQ_TEMPLATE = new Uint8Array([0, 1, 2, 3, 4, 5, 6, 7]);
 
         beforeEach(function () {
             sock = new Websock();
@@ -35,8 +35,8 @@ describe('Websock', function() {
 
         describe('rQpeek8', function () {
             it('should peek at the next byte without poping it off the queue', function () {
-                var bef_len = sock.rQlen();
-                var peek = sock.rQpeek8();
+                const bef_len = sock.rQlen();
+                const peek = sock.rQpeek8();
                 expect(sock.rQpeek8()).to.equal(peek);
                 expect(sock.rQlen()).to.equal(bef_len);
             });
@@ -44,8 +44,8 @@ describe('Websock', function() {
 
         describe('rQshift8', function () {
             it('should pop a single byte from the receive queue', function () {
-                var peek = sock.rQpeek8();
-                var bef_len = sock.rQlen();
+                const peek = sock.rQpeek8();
+                const bef_len = sock.rQlen();
                 expect(sock.rQshift8()).to.equal(peek);
                 expect(sock.rQlen()).to.equal(bef_len - 1);
             });
@@ -53,8 +53,8 @@ describe('Websock', function() {
 
         describe('rQshift16', function () {
             it('should pop two bytes from the receive queue and return a single number', function () {
-                var bef_len = sock.rQlen();
-                var expected = (RQ_TEMPLATE[0] << 8) + RQ_TEMPLATE[1];
+                const bef_len = sock.rQlen();
+                const expected = (RQ_TEMPLATE[0] << 8) + RQ_TEMPLATE[1];
                 expect(sock.rQshift16()).to.equal(expected);
                 expect(sock.rQlen()).to.equal(bef_len - 2);
             });
@@ -62,8 +62,8 @@ describe('Websock', function() {
 
         describe('rQshift32', function () {
             it('should pop four bytes from the receive queue and return a single number', function () {
-                var bef_len = sock.rQlen();
-                var expected = (RQ_TEMPLATE[0] << 24) +
+                const bef_len = sock.rQlen();
+                const expected = (RQ_TEMPLATE[0] << 24) +
                                (RQ_TEMPLATE[1] << 16) +
                                (RQ_TEMPLATE[2] << 8) +
                                RQ_TEMPLATE[3];
@@ -74,9 +74,9 @@ describe('Websock', function() {
 
         describe('rQshiftStr', function () {
             it('should shift the given number of bytes off of the receive queue and return a string', function () {
-                var bef_len = sock.rQlen();
-                var bef_rQi = sock.get_rQi();
-                var shifted = sock.rQshiftStr(3);
+                const bef_len = sock.rQlen();
+                const bef_rQi = sock.get_rQi();
+                const shifted = sock.rQshiftStr(3);
                 expect(shifted).to.be.a('string');
                 expect(shifted).to.equal(String.fromCharCode.apply(null, Array.prototype.slice.call(new Uint8Array(RQ_TEMPLATE.buffer, bef_rQi, 3))));
                 expect(sock.rQlen()).to.equal(bef_len - 3);
@@ -90,9 +90,9 @@ describe('Websock', function() {
 
         describe('rQshiftBytes', function () {
             it('should shift the given number of bytes of the receive queue and return an array', function () {
-                var bef_len = sock.rQlen();
-                var bef_rQi = sock.get_rQi();
-                var shifted = sock.rQshiftBytes(3);
+                const bef_len = sock.rQlen();
+                const bef_rQi = sock.get_rQi();
+                const shifted = sock.rQshiftBytes(3);
                 expect(shifted).to.be.an.instanceof(Uint8Array);
                 expect(shifted).to.array.equal(new Uint8Array(RQ_TEMPLATE.buffer, bef_rQi, 3));
                 expect(sock.rQlen()).to.equal(bef_len - 3);
@@ -110,19 +110,19 @@ describe('Websock', function() {
             });
 
             it('should not modify the receive queue', function () {
-                var bef_len = sock.rQlen();
+                const bef_len = sock.rQlen();
                 sock.rQslice(0, 2);
                 expect(sock.rQlen()).to.equal(bef_len);
             });
 
             it('should return an array containing the given slice of the receive queue', function () {
-                var sl = sock.rQslice(0, 2);
+                const sl = sock.rQslice(0, 2);
                 expect(sl).to.be.an.instanceof(Uint8Array);
                 expect(sl).to.array.equal(new Uint8Array(RQ_TEMPLATE.buffer, 0, 2));
             });
 
             it('should use the rest of the receive queue if no end is given', function () {
-                var sl = sock.rQslice(1);
+                const sl = sock.rQslice(1);
                 expect(sl).to.have.length(RQ_TEMPLATE.length - 1);
                 expect(sl).to.array.equal(new Uint8Array(RQ_TEMPLATE.buffer, 1));
             });
@@ -176,7 +176,7 @@ describe('Websock', function() {
                 sock._websocket.readyState = WebSocket.OPEN
                 sock._sQ = new Uint8Array([1, 2, 3]);
                 sock._sQlen = 3;
-                var encoded = sock._encode_message();
+                const encoded = sock._encode_message();
 
                 sock.flush();
                 expect(sock._websocket.send).to.have.been.calledOnce;
@@ -200,7 +200,7 @@ describe('Websock', function() {
 
             it('should add to the send queue', function () {
                 sock.send([1, 2, 3]);
-                var sq = sock.get_sQ();
+                const sq = sock.get_sQ();
                 expect(new Uint8Array(sq.buffer, sock._sQlen - 3, 3)).to.array.equal(new Uint8Array([1, 2, 3]));
             });
 
@@ -223,12 +223,12 @@ describe('Websock', function() {
     });
 
     describe('lifecycle methods', function () {
-        var old_WS;
+        let old_WS;
         before(function () {
            old_WS = WebSocket;
         });
 
-        var sock;
+        let sock;
         beforeEach(function () {
             sock = new Websock();
             // eslint-disable-next-line no-global-assign
@@ -333,14 +333,14 @@ describe('Websock', function() {
     });
 
     describe('WebSocket Receiving', function () {
-        var sock;
+        let sock;
         beforeEach(function () {
            sock = new Websock();
            sock._allocate_buffers();
         });
 
         it('should support adding binary Uint8Array data to the receive queue', function () {
-            var msg = { data: new Uint8Array([1, 2, 3]) };
+            const msg = { data: new Uint8Array([1, 2, 3]) };
             sock._mode = 'binary';
             sock._recv_message(msg);
             expect(sock.rQshiftStr(3)).to.equal('\x01\x02\x03');
@@ -348,7 +348,7 @@ describe('Websock', function() {
 
         it('should call the message event handler if present', function () {
             sock._eventHandlers.message = sinon.spy();
-            var msg = { data: new Uint8Array([1, 2, 3]).buffer };
+            const msg = { data: new Uint8Array([1, 2, 3]).buffer };
             sock._mode = 'binary';
             sock._recv_message(msg);
             expect(sock._eventHandlers.message).to.have.been.calledOnce;
@@ -356,7 +356,7 @@ describe('Websock', function() {
 
         it('should not call the message event handler if there is nothing in the receive queue', function () {
             sock._eventHandlers.message = sinon.spy();
-            var msg = { data: new Uint8Array([]).buffer };
+            const msg = { data: new Uint8Array([]).buffer };
             sock._mode = 'binary';
             sock._recv_message(msg);
             expect(sock._eventHandlers.message).not.to.have.been.called;
@@ -369,7 +369,7 @@ describe('Websock', function() {
             sock._rQlen = 6;
             sock.set_rQi(6);
             sock._rQmax = 3;
-            var msg = { data: new Uint8Array([1, 2, 3]).buffer };
+            const msg = { data: new Uint8Array([1, 2, 3]).buffer };
             sock._mode = 'binary';
             sock._recv_message(msg);
             expect(sock._rQlen).to.equal(3);
@@ -382,7 +382,7 @@ describe('Websock', function() {
             sock.set_rQi(0);
             sock._rQbufferSize = 20;
             sock._rQmax = 2;
-            var msg = { data: new Uint8Array(30).buffer };
+            const msg = { data: new Uint8Array(30).buffer };
             sock._mode = 'binary';
             sock._recv_message(msg);
             expect(sock._rQlen).to.equal(30);
@@ -396,7 +396,7 @@ describe('Websock', function() {
         after(function () { FakeWebSocket.restore(); });
 
         describe('as binary data', function () {
-            var sock;
+            let sock;
             beforeEach(function () {
                 sock = new Websock();
                 sock.open('ws://', 'binary');
@@ -406,7 +406,7 @@ describe('Websock', function() {
             it('should only send the send queue up to the send queue length', function () {
                 sock._sQ = new Uint8Array([1, 2, 3, 4, 5]);
                 sock._sQlen = 3;
-                var res = sock._encode_message();
+                const res = sock._encode_message();
                 expect(res).to.array.equal(new Uint8Array([1, 2, 3]));
             });
 
diff --git a/tests/test.webutil.js b/tests/test.webutil.js
index 27f9825..90c4bd3 100644
--- a/tests/test.webutil.js
+++ b/tests/test.webutil.js
@@ -1,6 +1,6 @@
 /* jshint expr: true */
 
-var expect = chai.expect;
+const expect = chai.expect;
 
 import * as WebUtil from '../app/webutil.js';
 
@@ -12,7 +12,7 @@ describe('WebUtil', function() {
     describe('settings', function () {
 
         describe('localStorage', function() {
-            var chrome = window.chrome;
+            let chrome = window.chrome;
             before(function() {
                 chrome = window.chrome;
                 window.chrome = null;
@@ -21,7 +21,7 @@ describe('WebUtil', function() {
                 window.chrome = chrome;
             });
 
-            var origLocalStorage;
+            let origLocalStorage;
             beforeEach(function() {
                 origLocalStorage = Object.getOwnPropertyDescriptor(window, "localStorage");
                 if (origLocalStorage === undefined) {
@@ -110,8 +110,8 @@ describe('WebUtil', function() {
         });
 
         describe('chrome.storage', function() {
-            var chrome = window.chrome;
-            var settings = {};
+            let chrome = window.chrome;
+            let settings = {};
             before(function() {
                 chrome = window.chrome;
                 window.chrome = {
@@ -128,7 +128,7 @@ describe('WebUtil', function() {
                 window.chrome = chrome;
             });
 
-            var csSandbox = sinon.createSandbox();
+            const csSandbox = sinon.createSandbox();
 
             beforeEach(function() {
                 settings = {};
diff --git a/utils/genkeysymdef.js b/utils/genkeysymdef.js
index 8bce1ed..9d09775 100755
--- a/utils/genkeysymdef.js
+++ b/utils/genkeysymdef.js
@@ -8,13 +8,12 @@
 
 "use strict";
 
-var fs = require('fs');
+const fs = require('fs');
 
-var show_help = process.argv.length === 2;
-var filename;
+let show_help = process.argv.length === 2;
+let filename;
 
-var i;
-for (i = 2; i < process.argv.length; ++i) {
+for (let i = 2; i < process.argv.length; ++i) {
   switch (process.argv[i]) {
     case "--help":
     case "-h":
@@ -40,25 +39,25 @@ if (show_help) {
   process.exit(0);
 }
 
-var buf = fs.readFileSync(filename);
-var str = buf.toString('utf8');
+const buf = fs.readFileSync(filename);
+const str = buf.toString('utf8');
 
-var re = /^#define XK_([a-zA-Z_0-9]+)\s+0x([0-9a-fA-F]+)\s*(\/\*\s*(.*)\s*\*\/)?\s*$/m;
+const re = /^#define XK_([a-zA-Z_0-9]+)\s+0x([0-9a-fA-F]+)\s*(\/\*\s*(.*)\s*\*\/)?\s*$/m;
 
-var arr = str.split('\n');
+const arr = str.split('\n');
 
-var codepoints = {};
+const codepoints = {};
 
-for (i = 0; i < arr.length; ++i) {
-    var result = re.exec(arr[i]);
+for (let i = 0; i < arr.length; ++i) {
+    const result = re.exec(arr[i]);
     if (result){
-        var keyname = result[1];
-        var keysym = parseInt(result[2], 16);
-        var remainder = result[3];
+        const keyname = result[1];
+        const keysym = parseInt(result[2], 16);
+        const remainder = result[3];
 
-        var unicodeRes = /U\+([0-9a-fA-F]+)/.exec(remainder);
+        const unicodeRes = /U\+([0-9a-fA-F]+)/.exec(remainder);
         if (unicodeRes) {
-            var unicode = parseInt(unicodeRes[1], 16);
+            const unicode = parseInt(unicodeRes[1], 16);
             // The first entry is the preferred one
             if (!codepoints[unicode]){
                 codepoints[unicode] = { keysym: keysym, name: keyname };
@@ -67,7 +66,7 @@ for (i = 0; i < arr.length; ++i) {
     }
 }
 
-var out =
+let out =
 "/*\n" +
 " * Mapping from Unicode codepoints to X11/RFB keysyms\n" +
 " *\n" +
@@ -77,17 +76,17 @@ var out =
 "\n" +
 "/* Functions at the bottom */\n" +
 "\n" +
-"var codepoints = {\n";
+"const codepoints = {\n";
 
 function toHex(num) {
-    var s = num.toString(16);
+    let s = num.toString(16);
     if (s.length < 4) {
         s = ("0000" + s).slice(-4);
     }
     return "0x" + s;
 }
 
-for (var codepoint in codepoints) {
+for (let codepoint in codepoints) {
     codepoint = parseInt(codepoint);
 
     // Latin-1?
@@ -116,7 +115,7 @@ out +=
 "        }\n" +
 "\n" +
 "        // Lookup table (fairly random)\n" +
-"        var keysym = codepoints[u];\n" +
+"        const keysym = codepoints[u];\n" +
 "        if (keysym !== undefined) {\n" +
 "            return keysym;\n" +
 "        }\n" +
diff --git a/utils/use_require.js b/utils/use_require.js
index 96f9c92..2d45f61 100755
--- a/utils/use_require.js
+++ b/utils/use_require.js
@@ -1,10 +1,10 @@
 #!/usr/bin/env node
 
-var path = require('path');
-var program = require('commander');
-var fs = require('fs');
-var fse = require('fs-extra');
-var babel = require('babel-core');
+const path = require('path');
+const program = require('commander');
+const fs = require('fs');
+const fse = require('fs-extra');
+const babel = require('babel-core');
 
 const SUPPORTED_FORMATS = new Set(['amd', 'commonjs', 'systemjs', 'umd']);
 
@@ -44,8 +44,8 @@ no_copy_files.forEach((file) => no_transform_files.add(file));
 // util.promisify requires Node.js 8.x, so we have our own
 function promisify(original) {
     return function () {
-        let obj = this;
-        let args = Array.prototype.slice.call(arguments);
+        const obj = this;
+        const args = Array.prototype.slice.call(arguments);
         return new Promise((resolve, reject) => {
             original.apply(obj, args.concat((err, value) => {
                 if (err) return reject(err);
@@ -70,10 +70,10 @@ const babelTransformFile = promisify(babel.transformFile);
 
 // walkDir *recursively* walks directories trees,
 // calling the callback for all normal files found.
-var walkDir = function (base_path, cb, filter) {
+const walkDir = function (base_path, cb, filter) {
     return readdir(base_path)
     .then(files => {
-        let paths = files.map(filename => path.join(base_path, filename));
+        const paths = files.map(filename => path.join(base_path, filename));
         return Promise.all(paths.map((filepath) => {
             return lstat(filepath)
             .then(stats => {
@@ -87,20 +87,20 @@ var walkDir = function (base_path, cb, filter) {
     });
 };
 
-var transform_html = function (legacy_scripts, only_legacy) {
+const transform_html = function (legacy_scripts, only_legacy) {
     // write out the modified vnc.html file that works with the bundle
-    var src_html_path = path.resolve(__dirname, '..', 'vnc.html');
-    var out_html_path = path.resolve(paths.out_dir_base, 'vnc.html');
+    const src_html_path = path.resolve(__dirname, '..', 'vnc.html');
+    const out_html_path = path.resolve(paths.out_dir_base, 'vnc.html');
     return readFile(src_html_path)
     .then(contents_raw => {
-        var contents = contents_raw.toString();
+        let contents = contents_raw.toString();
 
-        var start_marker = '<!-- begin scripts -->\n';
-        var end_marker = '<!-- end scripts -->';
-        var start_ind = contents.indexOf(start_marker) + start_marker.length;
-        var end_ind = contents.indexOf(end_marker, start_ind);
+        const start_marker = '<!-- begin scripts -->\n';
+        const end_marker = '<!-- end scripts -->';
+        const start_ind = contents.indexOf(start_marker) + start_marker.length;
+        const end_ind = contents.indexOf(end_marker, start_ind);
 
-        var new_script = '';
+        let new_script = '';
 
         if (only_legacy) {
             // Only legacy version, so include things directly
@@ -141,7 +141,7 @@ var transform_html = function (legacy_scripts, only_legacy) {
     });
 }
 
-var make_lib_files = function (import_format, source_maps, with_app_dir, only_legacy) {
+const make_lib_files = function (import_format, source_maps, with_app_dir, only_legacy) {
     if (!import_format) {
         throw new Error("you must specify an import format to generate compiled noVNC libraries");
     } else if (!SUPPORTED_FORMATS.has(import_format)) {
@@ -161,8 +161,8 @@ var make_lib_files = function (import_format, source_maps, with_app_dir, only_le
         only_legacy = true;
     }
 
-    var in_path;
-    var out_path_base;
+    let in_path;
+    let out_path_base;
     if (with_app_dir) {
         out_path_base = paths.out_dir_base;
         in_path = paths.main;
@@ -178,7 +178,7 @@ var make_lib_files = function (import_format, source_maps, with_app_dir, only_le
 
     const outFiles = [];
 
-    var handleDir = (js_only, vendor_rewrite, in_path_base, filename) => Promise.resolve()
+    const handleDir = (js_only, vendor_rewrite, in_path_base, filename) => Promise.resolve()
     .then(() => {
         if (no_copy_files.has(filename)) return;
 
@@ -225,7 +225,8 @@ var make_lib_files = function (import_format, source_maps, with_app_dir, only_le
             return babelTransformFile(filename, opts)
             .then(res => {
                 console.log(`Writing ${legacy_path}`);
-                var {code, map} = res;
+                const {map} = res;
+                let {code} = res;
                 if (source_maps === true) {
                     // append URL for external source map
                     code += `\n//# sourceMappingURL=${path.basename(legacy_path)}.map\n`;
@@ -249,19 +250,19 @@ var make_lib_files = function (import_format, source_maps, with_app_dir, only_le
 
     Promise.resolve()
     .then(() => {
-        let handler = handleDir.bind(null, true, false, in_path || paths.main);
-        let filter = (filename, stats) => !no_copy_files.has(filename);
+        const handler = handleDir.bind(null, true, false, in_path || paths.main);
+        const filter = (filename, stats) => !no_copy_files.has(filename);
         return walkDir(paths.vendor, handler, filter);
     })
     .then(() => {
-        let handler = handleDir.bind(null, true, !in_path, in_path || paths.core);
-        let filter = (filename, stats) => !no_copy_files.has(filename);
+        const handler = handleDir.bind(null, true, !in_path, in_path || paths.core);
+        const filter = (filename, stats) => !no_copy_files.has(filename);
         return walkDir(paths.core, handler, filter);
     })
     .then(() => {
         if (!with_app_dir) return;
-        let handler = handleDir.bind(null, false, false, in_path);
-        let filter = (filename, stats) => !no_copy_files.has(filename);
+        const handler = handleDir.bind(null, false, false, in_path);
+        const filter = (filename, stats) => !no_copy_files.has(filename);
         return walkDir(paths.app, handler, filter);
     })
     .then(() => {
@@ -275,8 +276,8 @@ var make_lib_files = function (import_format, source_maps, with_app_dir, only_le
         console.log(`Writing ${out_app_path}`);
         return helper.appWriter(out_path_base, legacy_path_base, out_app_path)
         .then(extra_scripts => {
-            let rel_app_path = path.relative(out_path_base, out_app_path);
-            let legacy_scripts = extra_scripts.concat([rel_app_path]);
+            const rel_app_path = path.relative(out_path_base, out_app_path);
+            const legacy_scripts = extra_scripts.concat([rel_app_path]);
             transform_html(legacy_scripts, only_legacy);
         })
         .then(() => {
@@ -287,7 +288,7 @@ var make_lib_files = function (import_format, source_maps, with_app_dir, only_le
                 .then(() => {
                     // Try to clean up any empty directories if this
                     // was the last file in there
-                    let rmdir_r = dir => {
+                    const rmdir_r = dir => {
                         return rmdir(dir)
                         .then(() => rmdir_r(path.dirname(dir)))
                         .catch(() => {
diff --git a/utils/use_require_helpers.js b/utils/use_require_helpers.js
index 9bba453..532e8f1 100644
--- a/utils/use_require_helpers.js
+++ b/utils/use_require_helpers.js
@@ -1,12 +1,12 @@
 // writes helpers require for vnc.html (they should output app.js)
-var fs = require('fs');
-var path = require('path');
+const fs = require('fs');
+const path = require('path');
 
 // util.promisify requires Node.js 8.x, so we have our own
 function promisify(original) {
     return function () {
-        let obj = this;
-        let args = Array.prototype.slice.call(arguments);
+        const obj = this;
+        const args = Array.prototype.slice.call(arguments);
         return new Promise((resolve, reject) => {
             original.apply(obj, args.concat((err, value) => {
                 if (err) return reject(err);
@@ -22,12 +22,12 @@ module.exports = {
     'amd': {
         appWriter: (base_out_path, script_base_path, out_path) => {
             // setup for requirejs
-            let ui_path = path.relative(base_out_path,
+            const ui_path = path.relative(base_out_path,
                                         path.join(script_base_path, 'app', 'ui'));
             return writeFile(out_path, `requirejs(["${ui_path}"], function (ui) {});`)
             .then(() => {
                 console.log(`Please place RequireJS in ${path.join(script_base_path, 'require.js')}`);
-                let require_path = path.relative(base_out_path,
+                const require_path = path.relative(base_out_path,
                                                  path.join(script_base_path, 'require.js'))
                 return [ require_path ];
             });
@@ -40,8 +40,8 @@ module.exports = {
             opts.plugins.unshift("add-module-exports");
         },
         appWriter: (base_out_path, script_base_path, out_path) => {
-            var browserify = require('browserify');
-            var b = browserify(path.join(script_base_path, 'app/ui.js'), {});
+            const browserify = require('browserify');
+            const b = browserify(path.join(script_base_path, 'app/ui.js'), {});
             return promisify(b.bundle).call(b)
             .then((buf) => writeFile(out_path, buf))
             .then(() => []);
@@ -51,15 +51,15 @@ module.exports = {
     },
     'systemjs': {
         appWriter: (base_out_path, script_base_path, out_path) => {
-            let ui_path = path.relative(base_out_path,
+            const ui_path = path.relative(base_out_path,
                                         path.join(script_base_path, 'app', 'ui.js'));
             return writeFile(out_path, `SystemJS.import("${ui_path}");`)
             .then(() => {
                 console.log(`Please place SystemJS in ${path.join(script_base_path, 'system-production.js')}`);
                 // FIXME: Should probably be in the legacy directory
-                let promise_path = path.relative(base_out_path,
+                const promise_path = path.relative(base_out_path,
                                                  path.join(base_out_path, 'vendor', 'promise.js'))
-                let systemjs_path = path.relative(base_out_path,
+                const systemjs_path = path.relative(base_out_path,
                                                   path.join(script_base_path, 'system-production.js'))
                 return [ promise_path, systemjs_path ];
             });