1 files changed, 24 insertions, 0 deletions
diff --git a/lib/internal/awaitify.js b/lib/internal/awaitify.js
new file mode 100644
index 0000000..57628de
--- /dev/null
+++ b/lib/internal/awaitify.js
@@ -0,0 +1,24 @@
+// conditionally promisify a function.
+// only return a promise if a callback is omitted
+export default function awaitify (asyncFn, arity = asyncFn.length) {
+    if (!arity) throw new Error('arity is undefined')
+    function awaitable (...args) {
+        if (typeof args[arity - 1] === 'function') {
+            return asyncFn.apply(this, args)
+        }
+
+        return new Promise((resolve, reject) => {
+            args[arity - 1] = (err, ...cbArgs) => {
+                if (err) return reject(err)
+                resolve(cbArgs.length > 1 ? cbArgs : cbArgs[0])
+            }
+            asyncFn.apply(this, args)
+        })
+    }
+
+    Object.defineProperty(awaitable, 'name', {
+        value: `awaitable(${asyncFn.name})`
+    })
+
+    return awaitable
+}