Priority Queue: Invoke callback when flushing queue

packages/priority-queue/src/index.js

@@ -1,3 +1,8 @@
+/**
+ * Internal dependencies
+ */
+import requestIdleCallback from './request-idle-callback';
+
 /**
  * Enqueued callback to invoke once idle time permits.
  *
@@ -31,9 +36,6 @@
  * @property {WPPriorityQueueFlush} flush Flush queue for context.
  */
 
-/** @type {typeof window.requestIdleCallback|typeof window.requestAnimationFrame} */
-const requestIdleCallback = window.requestIdleCallback ? window.requestIdleCallback : window.requestAnimationFrame;
-
 /**
  * Creates a context-aware queue that only executes
  * the last task of a given context.

packages/priority-queue/src/request-idle-callback.js

@@ -0,0 +1,14 @@
+/**
+ * @return {typeof window.requestIdleCallback|typeof window.requestAnimationFrame|((callback:(timestamp:number)=>void)=>void)}
+ */
+export function createRequestIdleCallback() {
+	if ( typeof 'window' === undefined ) {
+		return ( callback ) => {
+			setTimeout( () => callback( Date.now() ), 0 );
+		};
+	}
+
+	return window.requestIdleCallback || window.requestAnimationFrame;
+}
+
+export default createRequestIdleCallback();

packages/priority-queue/src/test/index.js

@@ -0,0 +1,34 @@
+/**
+ * External dependencies
+ */
+import { EventEmitter } from 'events';
+
+/**
+ * Internal dependencies
+ */
+import { createQueue } from '../';
+import requestIdleCallback from '../request-idle-callback';
+
+const emitter = new EventEmitter();
+
+const tick = () => emitter.emit( 'tick' );
+
+jest.mock( '../request-idle-callback', () => jest.fn() );
+
+requestIdleCallback.mockImplementation( ( callback ) => {
+	emitter.once( 'tick', () => callback( Date.now() ) );
+} );
+
+describe( 'createQueue', () => {
+	let queue;
+	beforeEach( () => {
+		queue = createQueue();
+	} );
+
+	it( 'runs callback after processing waiting queue', () => {
+		const callback = jest.fn();
+		queue.add( {}, callback );
+		tick();
+		expect( callback ).toHaveBeenCalled();
+	} );
+} );