Enable frozen string for app/helpers/**/*.rb

Partially addresses #47424.

102 files changed, 321 insertions, 130 deletions

diff --git a/app/helpers/accounts_helper.rb b/app/helpers/accounts_helper.rb
index 5d27d30eaa3..a4f19480539 100644
--- a/app/helpers/accounts_helper.rb
+++ b/app/helpers/accounts_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module AccountsHelper
   def incoming_email_token_enabled?
     current_user.incoming_email_token && Gitlab::IncomingEmail.supports_issue_creation?
diff --git a/app/helpers/active_sessions_helper.rb b/app/helpers/active_sessions_helper.rb
index 97b6dac67c5..84aa1160f12 100644
--- a/app/helpers/active_sessions_helper.rb
+++ b/app/helpers/active_sessions_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module ActiveSessionsHelper
   # Maps a device type as defined in `ActiveSession` to an svg icon name and
   # outputs the icon html.
diff --git a/app/helpers/appearances_helper.rb b/app/helpers/appearances_helper.rb
index f48db024e3f..ed13c5cfdd6 100644
--- a/app/helpers/appearances_helper.rb
+++ b/app/helpers/appearances_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module AppearancesHelper
   def brand_title
     current_appearance&.title.presence || 'GitLab Community Edition'
diff --git a/app/helpers/application_helper.rb b/app/helpers/application_helper.rb
index 0190aa90763..bb7ae03313c 100644
--- a/app/helpers/application_helper.rb
+++ b/app/helpers/application_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'digest/md5'
 require 'uri'
 
@@ -106,11 +108,11 @@ module ApplicationHelper
   #
   # Returns an HTML-safe String
   def time_ago_with_tooltip(time, placement: 'top', html_class: '', short_format: false)
-    css_classes = short_format ? 'js-short-timeago' : 'js-timeago'
-    css_classes << " #{html_class}" unless html_class.blank?
+    css_classes = [short_format ? 'js-short-timeago' : 'js-timeago']
+    css_classes << html_class unless html_class.blank?
 
     element = content_tag :time, l(time, format: "%b %d, %Y"),
-      class: css_classes,
+      class: css_classes.join(' '),
       title: l(time.to_time.in_time_zone, format: :timeago_tooltip),
       datetime: time.to_time.getutc.iso8601,
       data: {
diff --git a/app/helpers/application_settings_helper.rb b/app/helpers/application_settings_helper.rb
index 684c84c3006..52ccd9a106d 100644
--- a/app/helpers/application_settings_helper.rb
+++ b/app/helpers/application_settings_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module ApplicationSettingsHelper
   extend self
 
@@ -73,12 +75,12 @@ module ApplicationSettingsHelper
   def oauth_providers_checkboxes
     button_based_providers.map do |source|
       disabled = Gitlab::CurrentSettings.disabled_oauth_sign_in_sources.include?(source.to_s)
-      css_class = 'btn'
-      css_class << ' active' unless disabled
+      css_class = ['btn']
+      css_class << 'active' unless disabled
       checkbox_name = 'application_setting[enabled_oauth_sign_in_sources][]'
       name = Gitlab::Auth::OAuth::Provider.label_for(source)
 
-      label_tag(checkbox_name, class: css_class) do
+      label_tag(checkbox_name, class: css_class.join(' ')) do
         check_box_tag(checkbox_name, source, !disabled,
                       autocomplete: 'off',
                       id: name.tr(' ', '_')) + name
diff --git a/app/helpers/auth_helper.rb b/app/helpers/auth_helper.rb
index 18f0979fc86..c17a54a6dff 100644
--- a/app/helpers/auth_helper.rb
+++ b/app/helpers/auth_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module AuthHelper
   PROVIDERS_WITH_ICONS = %w(twitter github gitlab bitbucket google_oauth2 facebook azure_oauth2 authentiq).freeze
   LDAP_PROVIDER = /\Aldap/
diff --git a/app/helpers/auto_devops_helper.rb b/app/helpers/auto_devops_helper.rb
index 7b076728685..62fc6fb279f 100644
--- a/app/helpers/auto_devops_helper.rb
+++ b/app/helpers/auto_devops_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module AutoDevopsHelper
   def show_auto_devops_callout?(project)
     Feature.get(:auto_devops_banner_disabled).off? &&
diff --git a/app/helpers/avatars_helper.rb b/app/helpers/avatars_helper.rb
index 494f785e305..321811a3ca3 100644
--- a/app/helpers/avatars_helper.rb
+++ b/app/helpers/avatars_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module AvatarsHelper
   def project_icon(project_id, options = {})
     source_icon(Project, project_id, options)
@@ -125,9 +127,9 @@ module AvatarsHelper
 
   def source_identicon(source, options = {})
     bg_key = (source.id % 7) + 1
-    options[:class] ||= ''
-    options[:class] << ' identicon'
-    options[:class] << " bg#{bg_key}"
+
+    options[:class] =
+      [*options[:class], "identicon bg#{bg_key}"].join(' ')
 
     content_tag(:div, class: options[:class].strip) do
       source.name[0, 1].upcase
diff --git a/app/helpers/award_emoji_helper.rb b/app/helpers/award_emoji_helper.rb
index 86b19368cfd..b97a95629f7 100644
--- a/app/helpers/award_emoji_helper.rb
+++ b/app/helpers/award_emoji_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module AwardEmojiHelper
   def toggle_award_url(awardable)
     return url_for([:toggle_award_emoji, awardable]) unless @project || awardable.is_a?(Note)
diff --git a/app/helpers/blame_helper.rb b/app/helpers/blame_helper.rb
index 089d9e3e387..82c74e2416d 100644
--- a/app/helpers/blame_helper.rb
+++ b/app/helpers/blame_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module BlameHelper
   def age_map_duration(blame_groups, project)
     now = Time.zone.now
diff --git a/app/helpers/blob_helper.rb b/app/helpers/blob_helper.rb
index 96f7415ae98..9cbd5b5f785 100644
--- a/app/helpers/blob_helper.rb
+++ b/app/helpers/blob_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module BlobHelper
   def highlight(blob_name, blob_content, repository: nil, plain: false)
     plain ||= blob_content.length > Blob::MAXIMUM_TEXT_HIGHLIGHT_SIZE
diff --git a/app/helpers/boards_helper.rb b/app/helpers/boards_helper.rb
index af878bcf9a0..e3b74f443f7 100644
--- a/app/helpers/boards_helper.rb
+++ b/app/helpers/boards_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module BoardsHelper
   def board
     @board ||= @board || @boards.first
diff --git a/app/helpers/branches_helper.rb b/app/helpers/branches_helper.rb
index 07b1fc3d7cf..eadf48205fc 100644
--- a/app/helpers/branches_helper.rb
+++ b/app/helpers/branches_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module BranchesHelper
   def project_branches
     options_for_select(@project.repository.branch_names, @project.default_branch)
diff --git a/app/helpers/breadcrumbs_helper.rb b/app/helpers/breadcrumbs_helper.rb
index e88fe6bcd7e..b067376cea0 100644
--- a/app/helpers/breadcrumbs_helper.rb
+++ b/app/helpers/breadcrumbs_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module BreadcrumbsHelper
   def add_to_breadcrumbs(text, link)
     @breadcrumbs_extra_links ||= []
diff --git a/app/helpers/broadcast_messages_helper.rb b/app/helpers/broadcast_messages_helper.rb
index 0a15c29cfb5..289cb44f1e8 100644
--- a/app/helpers/broadcast_messages_helper.rb
+++ b/app/helpers/broadcast_messages_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module BroadcastMessagesHelper
   def broadcast_message(message)
     return unless message.present?
@@ -8,18 +10,17 @@ module BroadcastMessagesHelper
   end
 
   def broadcast_message_style(broadcast_message)
-    style = ''
+    style = []
 
     if broadcast_message.color.present?
       style << "background-color: #{broadcast_message.color}"
-      style << '; ' if broadcast_message.font.present?
     end
 
     if broadcast_message.font.present?
       style << "color: #{broadcast_message.font}"
     end
 
-    style
+    style.join('; ')
   end
 
   def broadcast_message_status(broadcast_message)
diff --git a/app/helpers/builds_helper.rb b/app/helpers/builds_helper.rb
index 4ec63fdaffc..3c8caec3fe5 100644
--- a/app/helpers/builds_helper.rb
+++ b/app/helpers/builds_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module BuildsHelper
   def build_summary(build, skip: false)
     if build.has_trace?
@@ -12,10 +14,10 @@ module BuildsHelper
   end
 
   def sidebar_build_class(build, current_build)
-    build_class = ''
-    build_class += ' active' if build.id === current_build.id
-    build_class += ' retried' if build.retried?
-    build_class
+    build_class = []
+    build_class << 'active' if build.id === current_build.id
+    build_class << 'retried' if build.retried?
+    build_class.join(' ')
   end
 
   def javascript_build_options
diff --git a/app/helpers/button_helper.rb b/app/helpers/button_helper.rb
index 2b3fe57767c..7f071d55a6b 100644
--- a/app/helpers/button_helper.rb
+++ b/app/helpers/button_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module ButtonHelper
   # Output a "Copy to Clipboard" button
   #
diff --git a/app/helpers/calendar_helper.rb b/app/helpers/calendar_helper.rb
index c54b91b0ce5..ad4116fc3da 100644
--- a/app/helpers/calendar_helper.rb
+++ b/app/helpers/calendar_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module CalendarHelper
   def calendar_url_options
     { format: :ics,
diff --git a/app/helpers/ci_status_helper.rb b/app/helpers/ci_status_helper.rb
index 330959e536d..f8d36dce45d 100644
--- a/app/helpers/ci_status_helper.rb
+++ b/app/helpers/ci_status_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 ##
 # DEPRECATED
 #
diff --git a/app/helpers/clusters_helper.rb b/app/helpers/clusters_helper.rb
index 73049c74d80..a67c91b21d7 100644
--- a/app/helpers/clusters_helper.rb
+++ b/app/helpers/clusters_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module ClustersHelper
   def has_multiple_clusters?(project)
     false
diff --git a/app/helpers/commits_helper.rb b/app/helpers/commits_helper.rb
index 7a942c44ac4..d52cfd6e37a 100644
--- a/app/helpers/commits_helper.rb
+++ b/app/helpers/commits_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module CommitsHelper
   # Returns a link to the commit author. If the author has a matching user and
   # is a member of the current @project it will link to the team member page.
diff --git a/app/helpers/compare_helper.rb b/app/helpers/compare_helper.rb
index 2df5b5d1695..9ece8b0bc5b 100644
--- a/app/helpers/compare_helper.rb
+++ b/app/helpers/compare_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module CompareHelper
   def create_mr_button?(from = params[:from], to = params[:to], project = @project)
     from.present? &&
diff --git a/app/helpers/components_helper.rb b/app/helpers/components_helper.rb
index 8893209b314..d0ef86851ad 100644
--- a/app/helpers/components_helper.rb
+++ b/app/helpers/components_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module ComponentsHelper
   def gitlab_workhorse_version
     if request.headers['Gitlab-Workhorse'].present?
diff --git a/app/helpers/conversational_development_index_helper.rb b/app/helpers/conversational_development_index_helper.rb
index 1ff54415811..37e5bb325fb 100644
--- a/app/helpers/conversational_development_index_helper.rb
+++ b/app/helpers/conversational_development_index_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module ConversationalDevelopmentIndexHelper
   def score_level(score)
     if score < 33.33
diff --git a/app/helpers/count_helper.rb b/app/helpers/count_helper.rb
index 5cd98f40f78..e16223a82c9 100644
--- a/app/helpers/count_helper.rb
+++ b/app/helpers/count_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module CountHelper
   def approximate_count_with_delimiters(count_data, model)
     count = count_data[model]
diff --git a/app/helpers/dashboard_helper.rb b/app/helpers/dashboard_helper.rb
index 19aa55a8d49..463f4145bdd 100644
--- a/app/helpers/dashboard_helper.rb
+++ b/app/helpers/dashboard_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module DashboardHelper
   def assigned_issues_dashboard_path
     issues_dashboard_path(assignee_id: current_user.id)
diff --git a/app/helpers/defer_script_tag_helper.rb b/app/helpers/defer_script_tag_helper.rb
index e1567556e5e..d91c6d52683 100644
--- a/app/helpers/defer_script_tag_helper.rb
+++ b/app/helpers/defer_script_tag_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module DeferScriptTagHelper
   # Override the default ActionView `javascript_include_tag` helper to support page specific deferred loading
   def javascript_include_tag(*sources)
diff --git a/app/helpers/deploy_tokens_helper.rb b/app/helpers/deploy_tokens_helper.rb
index bd921322476..80a5bb44c69 100644
--- a/app/helpers/deploy_tokens_helper.rb
+++ b/app/helpers/deploy_tokens_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module DeployTokensHelper
   def expand_deploy_tokens_section?(deploy_token)
     deploy_token.persisted? ||
diff --git a/app/helpers/diff_helper.rb b/app/helpers/diff_helper.rb
index 1bb82fd8150..7684734c014 100644
--- a/app/helpers/diff_helper.rb
+++ b/app/helpers/diff_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module DiffHelper
   def mark_inline_diffs(old_line, new_line)
     old_diffs, new_diffs = Gitlab::Diff::InlineDiff.new(old_line, new_line).inline_diffs
@@ -39,7 +41,8 @@ module DiffHelper
     line_num_class = %w[diff-line-num unfold js-unfold]
     line_num_class << 'js-unfold-bottom' if bottom
 
-    html = ''
+    html = []
+
     if old_pos
       html << content_tag(:td, '...', class: [*line_num_class, 'old_line'], data: { linenumber: old_pos })
       html << content_tag(:td, text, class: [*content_line_class, 'left-side']) if view == :parallel
@@ -50,7 +53,7 @@ module DiffHelper
       html << content_tag(:td, text, class: [*content_line_class, ('right-side' if view == :parallel)])
     end
 
-    html.html_safe
+    html.join.html_safe
   end
 
   def diff_line_content(line)
@@ -215,9 +218,7 @@ module DiffHelper
   end
 
   def toggle_whitespace_link(url, options)
-    options[:class] ||= ''
-    options[:class] << ' btn btn-default'
-
+    options[:class] = [*options[:class], 'btn btn-default'].join(' ')
     link_to "#{hide_whitespace? ? 'Show' : 'Hide'} whitespace changes", url, class: options[:class]
   end
 
diff --git a/app/helpers/dropdowns_helper.rb b/app/helpers/dropdowns_helper.rb
index 5a2360b4661..4b6c5b215e8 100644
--- a/app/helpers/dropdowns_helper.rb
+++ b/app/helpers/dropdowns_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module DropdownsHelper
   def dropdown_tag(toggle_text, options: {}, &block)
     content_tag :div, class: "dropdown #{options[:wrapper_class] if options.key?(:wrapper_class)}" do
@@ -10,7 +12,7 @@ module DropdownsHelper
       dropdown_output = dropdown_toggle(toggle_text, data_attr, options)
 
       dropdown_output << content_tag(:div, class: "dropdown-menu dropdown-select #{options[:dropdown_class] if options.key?(:dropdown_class)}") do
-        output = ""
+        output = []
 
         if options.key?(:title)
           output << dropdown_title(options[:title])
@@ -31,8 +33,7 @@ module DropdownsHelper
         end
 
         output << dropdown_loading
-
-        output.html_safe
+        output.join.html_safe
       end
 
       dropdown_output.html_safe
@@ -50,7 +51,7 @@ module DropdownsHelper
 
   def dropdown_title(title, options: {})
     content_tag :div, class: "dropdown-title" do
-      title_output = ""
+      title_output = []
 
       if options.fetch(:back, false)
         title_output << content_tag(:button, class: "dropdown-title-button dropdown-menu-back", aria: { label: "Go back" }, type: "button") do
@@ -66,7 +67,7 @@ module DropdownsHelper
         end
       end
 
-      title_output.html_safe
+      title_output.join.html_safe
     end
   end
 
diff --git a/app/helpers/emails_helper.rb b/app/helpers/emails_helper.rb
index c86a26ac30f..2d2e89a2a50 100644
--- a/app/helpers/emails_helper.rb
+++ b/app/helpers/emails_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module EmailsHelper
   include AppearancesHelper
 
@@ -49,8 +51,8 @@ module EmailsHelper
 
   def reset_token_expire_message
     link_tag = link_to('request a new one', new_user_password_url(user_email: @user.email))
-    msg = "This link is valid for #{password_reset_token_valid_time}.  "
-    msg << "After it expires, you can #{link_tag}."
+    "This link is valid for #{password_reset_token_valid_time}.  " \
+    "After it expires, you can #{link_tag}."
   end
 
   def header_logo
diff --git a/app/helpers/emoji_helper.rb b/app/helpers/emoji_helper.rb
index 482f68f412b..51b7fd7f352 100644
--- a/app/helpers/emoji_helper.rb
+++ b/app/helpers/emoji_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module EmojiHelper
   def emoji_icon(*args)
     raw Gitlab::Emoji.gl_emoji_tag(*args)
diff --git a/app/helpers/environment_helper.rb b/app/helpers/environment_helper.rb
index 1e78a189c08..4b3ef2de701 100644
--- a/app/helpers/environment_helper.rb
+++ b/app/helpers/environment_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module EnvironmentHelper
   def environment_for_build(project, build)
     return unless build.environment
diff --git a/app/helpers/environments_helper.rb b/app/helpers/environments_helper.rb
index c005ecbb56b..7b22bc8f98f 100644
--- a/app/helpers/environments_helper.rb
+++ b/app/helpers/environments_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module EnvironmentsHelper
   def environments_list_data
     {
diff --git a/app/helpers/events_helper.rb b/app/helpers/events_helper.rb
index 269acf5b2e2..401e2dab379 100644
--- a/app/helpers/events_helper.rb
+++ b/app/helpers/events_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module EventsHelper
   ICON_NAMES_BY_EVENT_TYPE = {
     'pushed to' => 'commit',
diff --git a/app/helpers/explore_helper.rb b/app/helpers/explore_helper.rb
index f062a91a166..62be591ec47 100644
--- a/app/helpers/explore_helper.rb
+++ b/app/helpers/explore_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module ExploreHelper
   def filter_projects_path(options = {})
     exist_opts = {
diff --git a/app/helpers/external_wiki_helper.rb b/app/helpers/external_wiki_helper.rb
index 8cf890b74a8..e36d63b2946 100644
--- a/app/helpers/external_wiki_helper.rb
+++ b/app/helpers/external_wiki_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module ExternalWikiHelper
   def get_project_wiki_path(project)
     external_wiki_service = project.external_wiki
diff --git a/app/helpers/favicon_helper.rb b/app/helpers/favicon_helper.rb
index 3a5342a8d9d..4a809731d97 100644
--- a/app/helpers/favicon_helper.rb
+++ b/app/helpers/favicon_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module FaviconHelper
   def favicon_extension_whitelist
     FaviconUploader::EXTENSION_WHITELIST
diff --git a/app/helpers/form_helper.rb b/app/helpers/form_helper.rb
index 905e2002592..5705ee54cee 100644
--- a/app/helpers/form_helper.rb
+++ b/app/helpers/form_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module FormHelper
   def form_errors(model, type: 'form')
     return unless model.errors.any?
diff --git a/app/helpers/git_helper.rb b/app/helpers/git_helper.rb
index 8ab394384f3..5edc6dcf454 100644
--- a/app/helpers/git_helper.rb
+++ b/app/helpers/git_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module GitHelper
   def strip_gpg_signature(text)
     text.gsub(/-----BEGIN PGP SIGNATURE-----(.*)-----END PGP SIGNATURE-----/m, "")
diff --git a/app/helpers/gitlab_routing_helper.rb b/app/helpers/gitlab_routing_helper.rb
index 61e12b0f31e..04cf43be452 100644
--- a/app/helpers/gitlab_routing_helper.rb
+++ b/app/helpers/gitlab_routing_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # Shorter routing method for some project items
 module GitlabRoutingHelper
   extend ActiveSupport::Concern
diff --git a/app/helpers/graph_helper.rb b/app/helpers/graph_helper.rb
index 1022070ab6f..49b15cde009 100644
--- a/app/helpers/graph_helper.rb
+++ b/app/helpers/graph_helper.rb
@@ -1,12 +1,14 @@
+# frozen_string_literal: true
+
 module GraphHelper
   def refs(repo, commit)
-    refs = commit.ref_names(repo).join(' ')
+    refs = [commit.ref_names(repo).join(' ')]
 
     # append note count
     notes_count = @graph.notes[commit.id]
     refs << "[#{pluralize(notes_count, 'note')}]" if notes_count > 0
 
-    refs
+    refs.join
   end
 
   def parents_zip_spaces(parents, parent_spaces)
diff --git a/app/helpers/groups_helper.rb b/app/helpers/groups_helper.rb
index 5b51d2f2425..f573fd399a5 100644
--- a/app/helpers/groups_helper.rb
+++ b/app/helpers/groups_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module GroupsHelper
   def group_nav_link_paths
     %w[groups#projects groups#edit badges#index ci_cd#show ldap_group_links#index hooks#index audit_events#index pipeline_quota#index]
@@ -43,22 +45,22 @@ module GroupsHelper
 
   def group_title(group, name = nil, url = nil)
     @has_group_title = true
-    full_title = ''
+    full_title = []
 
     group.ancestors.reverse.each_with_index do |parent, index|
       if index > 0
         add_to_breadcrumb_dropdown(group_title_link(parent, hidable: false, show_avatar: true, for_dropdown: true), location: :before)
       else
-        full_title += breadcrumb_list_item group_title_link(parent, hidable: false)
+        full_title << breadcrumb_list_item(group_title_link(parent, hidable: false))
       end
     end
 
-    full_title += render "layouts/nav/breadcrumbs/collapsed_dropdown", location: :before, title: _("Show parent subgroups")
+    full_title << render("layouts/nav/breadcrumbs/collapsed_dropdown", location: :before, title: _("Show parent subgroups"))
 
-    full_title += breadcrumb_list_item group_title_link(group)
-    full_title += ' &middot; '.html_safe + link_to(simple_sanitize(name), url, class: 'group-path breadcrumb-item-text js-breadcrumb-item-text') if name
+    full_title << breadcrumb_list_item(group_title_link(group))
+    full_title << ' &middot; '.html_safe + link_to(simple_sanitize(name), url, class: 'group-path breadcrumb-item-text js-breadcrumb-item-text') if name
 
-    full_title.html_safe
+    full_title.join.html_safe
   end
 
   def projects_lfs_status(group)
@@ -138,15 +140,8 @@ module GroupsHelper
 
   def group_title_link(group, hidable: false, show_avatar: false, for_dropdown: false)
     link_to(group_path(group), class: "group-path #{'breadcrumb-item-text' unless for_dropdown} js-breadcrumb-item-text #{'hidable' if hidable}") do
-      output =
-        if (group.try(:avatar_url) || show_avatar) && !Rails.env.test?
-          group_icon(group, class: "avatar-tile", width: 15, height: 15)
-        else
-          ""
-        end
-
-      output << simple_sanitize(group.name)
-      output.html_safe
+      icon = group_icon(group, class: "avatar-tile", width: 15, height: 15) if (group.try(:avatar_url) || show_avatar) && !Rails.env.test?
+      [icon, simple_sanitize(group.name)].join.html_safe
     end
   end
 
diff --git a/app/helpers/hooks_helper.rb b/app/helpers/hooks_helper.rb
index 0a356ba55d2..c4b39939192 100644
--- a/app/helpers/hooks_helper.rb
+++ b/app/helpers/hooks_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module HooksHelper
   def link_to_test_hook(hook, trigger)
     path = case hook
diff --git a/app/helpers/icons_helper.rb b/app/helpers/icons_helper.rb
index a5612372aa6..037004327b9 100644
--- a/app/helpers/icons_helper.rb
+++ b/app/helpers/icons_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'json'
 
 module IconsHelper
@@ -47,9 +49,10 @@ module IconsHelper
       end
     end
 
-    css_classes = size ? "s#{size}" : ""
-    css_classes << " #{css_class}" unless css_class.blank?
-    content_tag(:svg, content_tag(:use, "", { "xlink:href" => "#{sprite_icon_path}##{icon_name}" } ), class: css_classes.empty? ? nil : css_classes)
+    css_classes = []
+    css_classes << "s#{size}" if size
+    css_classes << "#{css_class}" unless css_class.blank?
+    content_tag(:svg, content_tag(:use, "", { "xlink:href" => "#{sprite_icon_path}##{icon_name}" } ), class: css_classes.empty? ? nil : css_classes.join(' '))
   end
 
   def external_snippet_icon(name)
@@ -70,10 +73,10 @@ module IconsHelper
   end
 
   def spinner(text = nil, visible = false)
-    css_class = 'loading'
-    css_class << ' hide' unless visible
+    css_class = ['loading']
+    css_class << 'hide' unless visible
 
-    content_tag :div, class: css_class do
+    content_tag :div, class: css_class.join(' ') do
       icon('spinner spin') + text
     end
   end
@@ -97,9 +100,10 @@ module IconsHelper
         'globe'
       end
 
-    name << " fw" if fw
+    name = [name]
+    name << "fw" if fw
 
-    icon(name, options)
+    icon(name.join(' '), options)
   end
 
   def file_type_icon_class(type, mode, name)
diff --git a/app/helpers/import_helper.rb b/app/helpers/import_helper.rb
index c65f1565425..3d0eb3d0d51 100644
--- a/app/helpers/import_helper.rb
+++ b/app/helpers/import_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module ImportHelper
   include ::Gitlab::Utils::StrongMemoize
 
diff --git a/app/helpers/instance_configuration_helper.rb b/app/helpers/instance_configuration_helper.rb
index cee319f20bc..f695be32743 100644
--- a/app/helpers/instance_configuration_helper.rb
+++ b/app/helpers/instance_configuration_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module InstanceConfigurationHelper
   def instance_configuration_cell_html(value, &block)
     return '-' unless value.to_s.presence
diff --git a/app/helpers/issuables_helper.rb b/app/helpers/issuables_helper.rb
index 79449a30305..8396cfe0ac8 100644
--- a/app/helpers/issuables_helper.rb
+++ b/app/helpers/issuables_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module IssuablesHelper
   include GitlabRoutingHelper
 
@@ -167,8 +169,9 @@ module IssuablesHelper
   end
 
   def issuable_meta(issuable, project, text)
-    output = ""
+    output = []
     output << "Opened #{time_ago_with_tooltip(issuable.created_at)} by ".html_safe
+
     output << content_tag(:strong) do
       author_output = link_to_member(project, issuable.author, size: 24, mobile_classes: "d-none d-sm-inline", tooltip: true)
       author_output << link_to_member(project, issuable.author, size: 24, by_username: true, avatar: false, mobile_classes: "d-block d-sm-none")
@@ -185,7 +188,7 @@ module IssuablesHelper
     output << content_tag(:span, (issuable.task_status if issuable.tasks?), id: "task_status", class: "d-none d-sm-none d-md-inline-block")
     output << content_tag(:span, (issuable.task_status_short if issuable.tasks?), id: "task_status_short", class: "d-md-none")
 
-    output.html_safe
+    output.join.html_safe
   end
 
   def issuable_todo(issuable)
diff --git a/app/helpers/issues_helper.rb b/app/helpers/issues_helper.rb
index 5b27d1d9404..f7d448ea3a7 100644
--- a/app/helpers/issues_helper.rb
+++ b/app/helpers/issues_helper.rb
@@ -1,9 +1,11 @@
+# frozen_string_literal: true
+
 module IssuesHelper
   def issue_css_classes(issue)
-    classes = "issue"
-    classes << " closed" if issue.closed?
-    classes << " today" if issue.today?
-    classes
+    classes = ["issue"]
+    classes << "closed" if issue.closed?
+    classes << "today" if issue.today?
+    classes.join(' ')
   end
 
   # Returns an OpenStruct object suitable for use by <tt>options_from_collection_for_select</tt>
@@ -105,8 +107,8 @@ module IssuesHelper
   end
 
   def link_to_discussions_to_resolve(merge_request, single_discussion = nil)
-    link_text = merge_request.to_reference
-    link_text += " (discussion #{single_discussion.first_note.id})" if single_discussion
+    link_text = [merge_request.to_reference]
+    link_text << "(discussion #{single_discussion.first_note.id})" if single_discussion
 
     path = if single_discussion
              Gitlab::UrlBuilder.build(single_discussion.first_note)
@@ -115,7 +117,7 @@ module IssuesHelper
              project_merge_request_path(project, merge_request)
            end
 
-    link_to link_text, path
+    link_to link_text.join(' '), path
   end
 
   def show_new_issue_link?(project)
diff --git a/app/helpers/javascript_helper.rb b/app/helpers/javascript_helper.rb
index cd4075b340d..7cb6da26236 100644
--- a/app/helpers/javascript_helper.rb
+++ b/app/helpers/javascript_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module JavascriptHelper
   def page_specific_javascript_tag(js)
     javascript_include_tag asset_path(js)
diff --git a/app/helpers/kerberos_spnego_helper.rb b/app/helpers/kerberos_spnego_helper.rb
index f5b0aa7549a..c0eb8f83f56 100644
--- a/app/helpers/kerberos_spnego_helper.rb
+++ b/app/helpers/kerberos_spnego_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module KerberosSpnegoHelper
   def allow_basic_auth?
     true # different behavior in GitLab Enterprise Edition
diff --git a/app/helpers/labels_helper.rb b/app/helpers/labels_helper.rb
index c7df25cecef..6c51739ba1a 100644
--- a/app/helpers/labels_helper.rb
+++ b/app/helpers/labels_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module LabelsHelper
   extend self
   include ActionView::Helpers::TagHelper
diff --git a/app/helpers/lazy_image_tag_helper.rb b/app/helpers/lazy_image_tag_helper.rb
index 603b9438e35..ac987a04895 100644
--- a/app/helpers/lazy_image_tag_helper.rb
+++ b/app/helpers/lazy_image_tag_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module LazyImageTagHelper
   def placeholder_image
     "data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw=="
@@ -11,9 +13,11 @@ module LazyImageTagHelper
       options[:data] ||= {}
       options[:data][:src] = path_to_image(source)
 
-      options[:class] ||= ""
-      options[:class] << " lazy"
+      # options[:class] can be either String or Array.
+      klass_opts = Array.wrap(options[:class])
+      klass_opts << "lazy"
 
+      options[:class] = klass_opts.join(' ')
       source = placeholder_image
     end
 
diff --git a/app/helpers/markup_helper.rb b/app/helpers/markup_helper.rb
index 3adaa1366c0..f2cd676bb1b 100644
--- a/app/helpers/markup_helper.rb
+++ b/app/helpers/markup_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'nokogiri'
 
 module MarkupHelper
diff --git a/app/helpers/mattermost_helper.rb b/app/helpers/mattermost_helper.rb
index 27ff4051c8d..b211fe5076a 100644
--- a/app/helpers/mattermost_helper.rb
+++ b/app/helpers/mattermost_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module MattermostHelper
   def mattermost_teams_options(teams)
     teams.map do |team|
diff --git a/app/helpers/members_helper.rb b/app/helpers/members_helper.rb
index a3129cac2b1..5a21403bc5e 100644
--- a/app/helpers/members_helper.rb
+++ b/app/helpers/members_helper.rb
@@ -1,8 +1,10 @@
+# frozen_string_literal: true
+
 module MembersHelper
   def remove_member_message(member, user: nil)
     user = current_user if defined?(current_user)
+    text = 'Are you sure you want to'
 
-    text = 'Are you sure you want to '
     action =
       if member.request?
         if member.user == user
@@ -16,13 +18,12 @@ module MembersHelper
         "remove #{member.user.name} from"
       end
 
-    text << action << " the #{member.source.human_name} #{member.real_source_type.humanize(capitalize: false)}?"
+    "#{text} #{action} the #{member.source.human_name} #{member.real_source_type.humanize(capitalize: false)}?"
   end
 
   def remove_member_title(member)
-    text = " from #{member.real_source_type.humanize(capitalize: false)}"
-
-    text.prepend(member.request? ? 'Deny access request' : 'Remove user')
+    action = member.request? ? 'Deny access request' : 'Remove user'
+    "#{action} from #{member.real_source_type.humanize(capitalize: false)}"
   end
 
   def leave_confirmation_message(member_source)
@@ -32,9 +33,6 @@ module MembersHelper
 
   def filter_group_project_member_path(options = {})
     options = params.slice(:search, :sort).merge(options)
-
-    path = request.path
-    path << "?#{options.to_param}"
-    path
+    "#{request.path}?#{options.to_param}"
   end
 end
diff --git a/app/helpers/merge_requests_helper.rb b/app/helpers/merge_requests_helper.rb
index 097be8a0643..87af6fb08f0 100644
--- a/app/helpers/merge_requests_helper.rb
+++ b/app/helpers/merge_requests_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module MergeRequestsHelper
   def new_mr_path_from_push_event(event)
     target_project = event.project.default_merge_request_target
@@ -19,10 +21,10 @@ module MergeRequestsHelper
   end
 
   def mr_css_classes(mr)
-    classes = "merge-request"
-    classes << " closed" if mr.closed?
-    classes << " merged" if mr.merged?
-    classes
+    classes = ["merge-request"]
+    classes << "closed" if mr.closed?
+    classes << "merged" if mr.merged?
+    classes.join(' ')
   end
 
   def ci_build_details_path(merge_request)
diff --git a/app/helpers/milestones_helper.rb b/app/helpers/milestones_helper.rb
index 95da8f00aff..999143002bb 100644
--- a/app/helpers/milestones_helper.rb
+++ b/app/helpers/milestones_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module MilestonesHelper
   include EntityDateHelper
 
@@ -119,20 +121,18 @@ module MilestonesHelper
     title = date_type == :start ? "Start date" : "End date"
 
     if date
-      time_ago = time_ago_in_words(date)
-      time_ago.slice!("about ")
-
-      time_ago << if date.past?
-                    " ago"
-                  else
-                    " remaining"
-                  end
+      time_ago = time_ago_in_words(date).sub("about ", "")
+      state = if date.past?
+                "ago"
+              else
+                "remaining"
+              end
 
       content = [
         title,
         "<br />",
         date.to_s(:medium),
-        "(#{time_ago})"
+        "(#{time_ago} #{state})"
       ].join(" ")
 
       content.html_safe
diff --git a/app/helpers/milestones_routing_helper.rb b/app/helpers/milestones_routing_helper.rb
index a0b2616f224..a49b561533a 100644
--- a/app/helpers/milestones_routing_helper.rb
+++ b/app/helpers/milestones_routing_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module MilestonesRoutingHelper
   def milestone_path(milestone, *args)
     if milestone.group_milestone?
diff --git a/app/helpers/mirror_helper.rb b/app/helpers/mirror_helper.rb
index 93ed22513ac..a4025730397 100644
--- a/app/helpers/mirror_helper.rb
+++ b/app/helpers/mirror_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module MirrorHelper
   def mirrors_form_data_attributes
     { project_mirror_endpoint: project_mirror_path(@project) }
diff --git a/app/helpers/namespaces_helper.rb b/app/helpers/namespaces_helper.rb
index 6535afb6425..e7537ca5733 100644
--- a/app/helpers/namespaces_helper.rb
+++ b/app/helpers/namespaces_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module NamespacesHelper
   def namespace_id_from(params)
     params.dig(:project, :namespace_id) || params[:namespace_id]
diff --git a/app/helpers/nav_helper.rb b/app/helpers/nav_helper.rb
index a84a39235d8..761f42f2f0f 100644
--- a/app/helpers/nav_helper.rb
+++ b/app/helpers/nav_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module NavHelper
   def header_links
     @header_links ||= get_header_links
diff --git a/app/helpers/notes_helper.rb b/app/helpers/notes_helper.rb
index 5404ead44f3..eb2be8906ff 100644
--- a/app/helpers/notes_helper.rb
+++ b/app/helpers/notes_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module NotesHelper
   def note_target_fields(note)
     if note.noteable
diff --git a/app/helpers/notifications_helper.rb b/app/helpers/notifications_helper.rb
index a185f2916d4..5318ab4ddef 100644
--- a/app/helpers/notifications_helper.rb
+++ b/app/helpers/notifications_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module NotificationsHelper
   include IconsHelper
 
diff --git a/app/helpers/numbers_helper.rb b/app/helpers/numbers_helper.rb
index 45bd3606076..f609b6c0cec 100644
--- a/app/helpers/numbers_helper.rb
+++ b/app/helpers/numbers_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module NumbersHelper
   def limited_counter_with_delimiter(resource, **options)
     limit = options.fetch(:limit, 1000).to_i
diff --git a/app/helpers/page_layout_helper.rb b/app/helpers/page_layout_helper.rb
index 68d892393ef..b33c074d1af 100644
--- a/app/helpers/page_layout_helper.rb
+++ b/app/helpers/page_layout_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module PageLayoutHelper
   def page_title(*titles)
     @page_title ||= []
@@ -65,14 +67,14 @@ module PageLayoutHelper
   end
 
   def page_card_meta_tags
-    tags = ''
+    tags = []
 
     page_card_attributes.each_with_index do |pair, i|
       tags << tag(:meta, property: "twitter:label#{i + 1}", content: pair[0])
       tags << tag(:meta, property: "twitter:data#{i + 1}",  content: pair[1])
     end
 
-    tags.html_safe
+    tags.join.html_safe
   end
 
   def header_title(title = nil, title_url = nil)
@@ -115,16 +117,16 @@ module PageLayoutHelper
   end
 
   def container_class
-    css_class = "container-fluid"
+    css_class = ["container-fluid"]
 
     unless fluid_layout
-      css_class += " container-limited"
+      css_class << "container-limited"
     end
 
     if blank_container
-      css_class += " container-blank"
+      css_class << "container-blank"
     end
 
-    css_class
+    css_class.join(' ')
   end
 end
diff --git a/app/helpers/pagination_helper.rb b/app/helpers/pagination_helper.rb
index 83dd76a01dd..d05153c9d4b 100644
--- a/app/helpers/pagination_helper.rb
+++ b/app/helpers/pagination_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module PaginationHelper
   def paginate_collection(collection, remote: nil)
     if collection.is_a?(Kaminari::PaginatableWithoutCount)
diff --git a/app/helpers/performance_bar_helper.rb b/app/helpers/performance_bar_helper.rb
index d24efe37f5f..7518cec160c 100644
--- a/app/helpers/performance_bar_helper.rb
+++ b/app/helpers/performance_bar_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module PerformanceBarHelper
   # This is a hack since using `alias_method :performance_bar_enabled?, :peek_enabled?`
   # in WithPerformanceBar breaks tests (but works in the browser).
diff --git a/app/helpers/pipeline_schedules_helper.rb b/app/helpers/pipeline_schedules_helper.rb
index 4b9f6bd2caf..0e166106b32 100644
--- a/app/helpers/pipeline_schedules_helper.rb
+++ b/app/helpers/pipeline_schedules_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module PipelineSchedulesHelper
   def timezone_data
     ActiveSupport::TimeZone.all.map do |timezone|
diff --git a/app/helpers/preferences_helper.rb b/app/helpers/preferences_helper.rb
index fb523cb865b..ff9842d4cd9 100644
--- a/app/helpers/preferences_helper.rb
+++ b/app/helpers/preferences_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # Helper methods for per-User preferences
 module PreferencesHelper
   def layout_choices
diff --git a/app/helpers/profiles_helper.rb b/app/helpers/profiles_helper.rb
index e7aa92e6e5c..55674e37a34 100644
--- a/app/helpers/profiles_helper.rb
+++ b/app/helpers/profiles_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module ProfilesHelper
   def attribute_provider_label(attribute)
     user_synced_attributes_metadata = current_user.user_synced_attributes_metadata
diff --git a/app/helpers/projects_helper.rb b/app/helpers/projects_helper.rb
index 7a9b63d8852..89fee06ee77 100644
--- a/app/helpers/projects_helper.rb
+++ b/app/helpers/projects_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module ProjectsHelper
   def link_to_project(project)
     link_to [project.namespace.becomes(Namespace), project], title: h(project.name) do
@@ -50,7 +52,7 @@ module ProjectsHelper
 
     return "(deleted)" unless author
 
-    author_html = ""
+    author_html = []
 
     # Build avatar image tag
     author_html << link_to_member_avatar(author, opts) if opts[:avatar]
@@ -60,7 +62,7 @@ module ProjectsHelper
 
     author_html << capture(&block) if block
 
-    author_html = author_html.html_safe
+    author_html = author_html.join.html_safe
 
     if opts[:name]
       link_to(author_html, user_path(author), class: "author-link #{"#{opts[:extra_class]}" if opts[:extra_class]} #{"#{opts[:mobile_classes]}" if opts[:mobile_classes]}").html_safe
@@ -80,15 +82,8 @@ module ProjectsHelper
       end
 
     project_link = link_to project_path(project) do
-      output =
-        if project.avatar_url && !Rails.env.test?
-          project_icon(project, alt: project.name, class: 'avatar-tile', width: 15, height: 15)
-        else
-          ""
-        end
-
-      output << content_tag("span", simple_sanitize(project.name), class: "breadcrumb-item-text js-breadcrumb-item-text")
-      output.html_safe
+      icon = project_icon(project, alt: project.name, class: 'avatar-tile', width: 15, height: 15) if project.avatar_url && !Rails.env.test?
+      [icon, content_tag("span", simple_sanitize(project.name), class: "breadcrumb-item-text js-breadcrumb-item-text")].join.html_safe
     end
 
     namespace_link = breadcrumb_list_item(namespace_link) unless project.group
diff --git a/app/helpers/repository_languages_helper.rb b/app/helpers/repository_languages_helper.rb
index 9a842cf5ce0..c1505b52808 100644
--- a/app/helpers/repository_languages_helper.rb
+++ b/app/helpers/repository_languages_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RepositoryLanguagesHelper
   def repository_languages_bar(languages)
     return if languages.none?
diff --git a/app/helpers/rss_helper.rb b/app/helpers/rss_helper.rb
index 7d4fa83a67a..67c7d244f11 100644
--- a/app/helpers/rss_helper.rb
+++ b/app/helpers/rss_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RssHelper
   def rss_url_options
     { format: :atom, feed_token: current_user.try(:feed_token) }
diff --git a/app/helpers/runners_helper.rb b/app/helpers/runners_helper.rb
index 9fb42487a75..cb21f922401 100644
--- a/app/helpers/runners_helper.rb
+++ b/app/helpers/runners_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RunnersHelper
   def runner_status_icon(runner)
     status = runner.status
diff --git a/app/helpers/safe_params_helper.rb b/app/helpers/safe_params_helper.rb
index b568e8810cc..72bf1377b02 100644
--- a/app/helpers/safe_params_helper.rb
+++ b/app/helpers/safe_params_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module SafeParamsHelper
   # Rails 5.0 requires to permit `params` if they're used in url helpers.
   # Use this helper when generating links with `params.merge(...)`
diff --git a/app/helpers/search_helper.rb b/app/helpers/search_helper.rb
index 98074a4c0c5..c509cd592c4 100644
--- a/app/helpers/search_helper.rb
+++ b/app/helpers/search_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module SearchHelper
   def search_autocomplete_opts(term)
     return unless current_user
diff --git a/app/helpers/selects_helper.rb b/app/helpers/selects_helper.rb
index 6cefcde558a..cf60696ef39 100644
--- a/app/helpers/selects_helper.rb
+++ b/app/helpers/selects_helper.rb
@@ -1,12 +1,14 @@
+# frozen_string_literal: true
+
 module SelectsHelper
   def users_select_tag(id, opts = {})
-    css_class = "ajax-users-select "
-    css_class << "multiselect " if opts[:multiple]
-    css_class << "skip_ldap " if opts[:skip_ldap]
+    css_class = ["ajax-users-select"]
+    css_class << "multiselect" if opts[:multiple]
+    css_class << "skip_ldap" if opts[:skip_ldap]
     css_class << (opts[:class] || '')
     value = opts[:selected] || ''
     html = {
-      class: css_class,
+      class: css_class.join(' '),
       data: users_select_data_attributes(opts)
     }
 
@@ -24,20 +26,21 @@ module SelectsHelper
   end
 
   def groups_select_tag(id, opts = {})
-    opts[:class] ||= ''
-    opts[:class] << ' ajax-groups-select'
+    classes = Array.wrap(opts[:class])
+    classes << 'ajax-groups-select'
+
+    opts[:class] = classes.join(' ')
+
     select2_tag(id, opts)
   end
 
   def namespace_select_tag(id, opts = {})
-    opts[:class] ||= ''
-    opts[:class] << ' ajax-namespace-select'
+    opts[:class] = [*opts[:class], 'ajax-namespace-select'].join(' ')
     select2_tag(id, opts)
   end
 
   def project_select_tag(id, opts = {})
-    opts[:class] ||= ''
-    opts[:class] << ' ajax-project-select'
+    opts[:class] = [*opts[:class], 'ajax-project-select'].join(' ')
 
     unless opts.delete(:scope) == :all
       if @group
@@ -57,7 +60,10 @@ module SelectsHelper
   end
 
   def select2_tag(id, opts = {})
-    opts[:class] << ' multiselect' if opts[:multiple]
+    klass_opts = [opts[:class]]
+    klass_opts << 'multiselect' if opts[:multiple]
+
+    opts[:class] = klass_opts.join(' ')
     value = opts[:selected] || ''
 
     hidden_field_tag(id, value, opts)
diff --git a/app/helpers/sentry_helper.rb b/app/helpers/sentry_helper.rb
index 3d255df66a0..d53eaef9952 100644
--- a/app/helpers/sentry_helper.rb
+++ b/app/helpers/sentry_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module SentryHelper
   def sentry_enabled?
     Gitlab::Sentry.enabled?
diff --git a/app/helpers/services_helper.rb b/app/helpers/services_helper.rb
index f872990122e..8b554e1aaa9 100644
--- a/app/helpers/services_helper.rb
+++ b/app/helpers/services_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module ServicesHelper
   def service_event_description(event)
     case event
diff --git a/app/helpers/sidekiq_helper.rb b/app/helpers/sidekiq_helper.rb
index 50aeb7f4b82..32bf3526571 100644
--- a/app/helpers/sidekiq_helper.rb
+++ b/app/helpers/sidekiq_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module SidekiqHelper
   SIDEKIQ_PS_REGEXP = %r{\A
     (?<pid>\d+)\s+
diff --git a/app/helpers/snippets_helper.rb b/app/helpers/snippets_helper.rb
index a05640773ad..c7d31f3469d 100644
--- a/app/helpers/snippets_helper.rb
+++ b/app/helpers/snippets_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module SnippetsHelper
   def reliable_snippet_path(snippet, opts = nil)
     if snippet.project_id?
diff --git a/app/helpers/sorting_helper.rb b/app/helpers/sorting_helper.rb
index 36a311dfa8a..731b6806b5f 100644
--- a/app/helpers/sorting_helper.rb
+++ b/app/helpers/sorting_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module SortingHelper
   def sort_options_hash
     {
diff --git a/app/helpers/storage_health_helper.rb b/app/helpers/storage_health_helper.rb
index b76c1228220..182e8e6641b 100644
--- a/app/helpers/storage_health_helper.rb
+++ b/app/helpers/storage_health_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module StorageHealthHelper
   def failing_storage_health_message(storage_health)
     storage_name = content_tag(:strong, h(storage_health.storage_name))
diff --git a/app/helpers/storage_helper.rb b/app/helpers/storage_helper.rb
index e19c67a37ca..be8761db562 100644
--- a/app/helpers/storage_helper.rb
+++ b/app/helpers/storage_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module StorageHelper
   def storage_counter(size_in_bytes)
     precision = size_in_bytes < 1.megabyte ? 0 : 1
diff --git a/app/helpers/submodule_helper.rb b/app/helpers/submodule_helper.rb
index ec2cf2b16c0..164c69ca50b 100644
--- a/app/helpers/submodule_helper.rb
+++ b/app/helpers/submodule_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module SubmoduleHelper
   extend self
 
diff --git a/app/helpers/system_note_helper.rb b/app/helpers/system_note_helper.rb
index 5b4a141dbcf..8ed8f387bb4 100644
--- a/app/helpers/system_note_helper.rb
+++ b/app/helpers/system_note_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module SystemNoteHelper
   ICON_NAMES_BY_ACTION = {
     'commit' => 'commit',
diff --git a/app/helpers/tab_helper.rb b/app/helpers/tab_helper.rb
index ee701076a14..e310fda51d7 100644
--- a/app/helpers/tab_helper.rb
+++ b/app/helpers/tab_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module TabHelper
   # Navigation link helper
   #
@@ -47,9 +49,7 @@ module TabHelper
     # Add our custom class into the html_options, which may or may not exist
     # and which may or may not already have a :class key
     o = options.delete(:html_options) || {}
-    o[:class] ||= ''
-    o[:class] += ' ' + klass
-    o[:class].strip!
+    o[:class] = [*o[:class], klass].join(' ').strip
 
     if block_given?
       content_tag(:li, capture(&block), o)
diff --git a/app/helpers/tags_helper.rb b/app/helpers/tags_helper.rb
index d000d6b1c0a..de0b92b6fd7 100644
--- a/app/helpers/tags_helper.rb
+++ b/app/helpers/tags_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module TagsHelper
   def tag_path(tag)
     "/tags/#{tag}"
@@ -14,12 +16,13 @@ module TagsHelper
   end
 
   def tag_list(project)
-    html = ''
+    html = []
+
     project.tag_list.each do |tag|
       html << link_to(tag, tag_path(tag))
     end
 
-    html.html_safe
+    html.join.html_safe
   end
 
   def protected_tag?(project, tag)
diff --git a/app/helpers/time_helper.rb b/app/helpers/time_helper.rb
index 336385f6798..94044d7b85e 100644
--- a/app/helpers/time_helper.rb
+++ b/app/helpers/time_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module TimeHelper
   def time_interval_in_words(interval_in_seconds)
     interval_in_seconds = interval_in_seconds.to_i
diff --git a/app/helpers/todos_helper.rb b/app/helpers/todos_helper.rb
index 7cd74358168..6bd78336ed3 100644
--- a/app/helpers/todos_helper.rb
+++ b/app/helpers/todos_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module TodosHelper
   def todos_pending_count
     @todos_pending_count ||= current_user.todos_pending_count
@@ -94,9 +96,7 @@ module TodosHelper
       end
     end
 
-    path = request.path
-    path << "?#{options.to_param}"
-    path
+    "#{request.path}?#{options.to_param}"
   end
 
   def todo_actions_options
@@ -152,10 +152,11 @@ module TodosHelper
         ''
       end
 
-    html = "&middot; ".html_safe
-    html << content_tag(:span, class: css_class) do
+    content = content_tag(:span, class: css_class) do
       "Due #{is_due_today ? "today" : todo.target.due_date.to_s(:medium)}"
     end
+
+    "&middot; #{content}".html_safe
   end
 
   private
diff --git a/app/helpers/tree_helper.rb b/app/helpers/tree_helper.rb
index dc42caa70e5..80f61a371fd 100644
--- a/app/helpers/tree_helper.rb
+++ b/app/helpers/tree_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module TreeHelper
   FILE_LIMIT = 1_000
 
@@ -8,7 +10,7 @@ module TreeHelper
   def render_tree(tree)
     # Sort submodules and folders together by name ahead of files
     folders, files, submodules = tree.trees, tree.blobs, tree.submodules
-    tree = ''
+    tree = []
     items = (folders + submodules).sort_by(&:name) + files
 
     if items.size > FILE_LIMIT
@@ -18,7 +20,7 @@ module TreeHelper
     end
 
     tree << render(partial: 'projects/tree/tree_row', collection: items) if items.present?
-    tree.html_safe
+    tree.join.html_safe
   end
 
   # Return an image icon depending on the file type and mode
diff --git a/app/helpers/triggers_helper.rb b/app/helpers/triggers_helper.rb
index ce435ca2241..5cfdc0971f0 100644
--- a/app/helpers/triggers_helper.rb
+++ b/app/helpers/triggers_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module TriggersHelper
   def builds_trigger_url(project_id, ref: nil)
     if ref.nil?
diff --git a/app/helpers/user_callouts_helper.rb b/app/helpers/user_callouts_helper.rb
index 657a3227dd4..bae01d476df 100644
--- a/app/helpers/user_callouts_helper.rb
+++ b/app/helpers/user_callouts_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module UserCalloutsHelper
   GKE_CLUSTER_INTEGRATION = 'gke_cluster_integration'.freeze
   GCP_SIGNUP_OFFER = 'gcp_signup_offer'.freeze
diff --git a/app/helpers/users_helper.rb b/app/helpers/users_helper.rb
index 2c0c4254a0c..bcd91f619c8 100644
--- a/app/helpers/users_helper.rb
+++ b/app/helpers/users_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module UsersHelper
   def user_link(user)
     link_to(user.name, user_path(user),
diff --git a/app/helpers/version_check_helper.rb b/app/helpers/version_check_helper.rb
index c20753ece72..fd003396230 100644
--- a/app/helpers/version_check_helper.rb
+++ b/app/helpers/version_check_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module VersionCheckHelper
   def version_status_badge
     if Rails.env.production? && Gitlab::CurrentSettings.version_check_enabled
diff --git a/app/helpers/visibility_level_helper.rb b/app/helpers/visibility_level_helper.rb
index 7b64869c9ea..e690350a0d1 100644
--- a/app/helpers/visibility_level_helper.rb
+++ b/app/helpers/visibility_level_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module VisibilityLevelHelper
   def visibility_level_color(level)
     case level
@@ -82,7 +84,7 @@ module VisibilityLevelHelper
   def disallowed_project_visibility_level_description(level, project)
     level_name = Gitlab::VisibilityLevel.level_name(level).downcase
     reasons = []
-    instructions = ''
+    instructions = []
 
     unless project.visibility_level_allowed_as_fork?(level)
       reasons << "the fork source project has lower visibility"
@@ -96,7 +98,7 @@ module VisibilityLevelHelper
     end
 
     reasons = reasons.any? ? ' because ' + reasons.to_sentence : ''
-    "This project cannot be #{level_name}#{reasons}.#{instructions}".html_safe
+    "This project cannot be #{level_name}#{reasons}.#{instructions.join}".html_safe
   end
 
   # Note: these messages closely mirror the form validation strings found in the group
@@ -104,7 +106,7 @@ module VisibilityLevelHelper
   def disallowed_group_visibility_level_description(level, group)
     level_name = Gitlab::VisibilityLevel.level_name(level).downcase
     reasons = []
-    instructions = ''
+    instructions = []
 
     unless group.visibility_level_allowed_by_projects?(level)
       reasons << "it contains projects with higher visibility"
@@ -122,7 +124,7 @@ module VisibilityLevelHelper
     end
 
     reasons = reasons.any? ? ' because ' + reasons.to_sentence : ''
-    "This group cannot be #{level_name}#{reasons}.#{instructions}".html_safe
+    "This group cannot be #{level_name}#{reasons}.#{instructions.join}".html_safe
   end
 
   def visibility_icon_description(form_model)
diff --git a/app/helpers/webpack_helper.rb b/app/helpers/webpack_helper.rb
index 72f6b397046..345ddcf023a 100644
--- a/app/helpers/webpack_helper.rb
+++ b/app/helpers/webpack_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module WebpackHelper
   def webpack_bundle_tag(bundle)
     javascript_include_tag(*webpack_entrypoint_paths(bundle))
diff --git a/app/helpers/wiki_helper.rb b/app/helpers/wiki_helper.rb
index 17940aeb900..647f34e57ed 100644
--- a/app/helpers/wiki_helper.rb
+++ b/app/helpers/wiki_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module WikiHelper
   include API::Helpers::RelatedResourcesHelpers
 
diff --git a/app/helpers/workhorse_helper.rb b/app/helpers/workhorse_helper.rb
index fd1d78bd9b8..f19445fca1a 100644
--- a/app/helpers/workhorse_helper.rb
+++ b/app/helpers/workhorse_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # Helpers to send Git blobs, diffs, patches or archives through Workhorse.
 # Workhorse will also serve files when using `send_file`.
 module WorkhorseHelper
diff --git a/app/models/license_template.rb b/app/models/license_template.rb
index 0ad75b27827..693a6a89fd2 100644
--- a/app/models/license_template.rb
+++ b/app/models/license_template.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 class LicenseTemplate
   PROJECT_TEMPLATE_REGEX =
     %r{[\<\{\[]
diff --git a/changelogs/unreleased/frozen-string-enable-app-helpers.yml b/changelogs/unreleased/frozen-string-enable-app-helpers.yml
new file mode 100644
index 00000000000..7f6805ccb5a
--- /dev/null
+++ b/changelogs/unreleased/frozen-string-enable-app-helpers.yml
@@ -0,0 +1,5 @@
+---
+title: Enable frozen string for app/helpers/**/*.rb
+merge_request:
+author: gfyoung
+type: performance