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Testing Async JavaScript with Mocha & Chai

Writing unit tests for asynchronous JavaScript can present issues. Unit testing frameworks that are around now can handle Promises, allowing us to write unit tests that asynchronously fetch data. Async DOM manipulation however is often not explicitly discussed – perhaps unit testing DOM manipulation is a fools errand? Nonetheless I spent some time figuring out how to write a unit test for a piece of JavaScript that used RequestAnimationFrame when doing DOM manipulation.

A few years ago I wrote about managing unit testing workflow using Karma, using Jasmine as my go-to framework. I went on a bit of a journey and it turns out that Jasmine does have asynchronous support, but the examples below were achieved with Mocha and Chai (although they basically read pretty much exactly the same).

Mocha makes writing asynchronous unit tests really easy by adding callback support. It also has great Promise support, and if that's not enough you can get an even nicer syntax using Chai and the Chai as Promised package.

After finding out about all of that, it turned out that I just needed to use callbacks when writing async DOM manipulation unit tests. Here's an example test for my "bleepBloop" module, which outputs passed strings into the DOM asynchronously using RequestAnimationFrame.

describe('bleep bloop', function() {
    beforeEach(function() {
        // Mock DOM here

    it('should async DOM manipulate', function(done) {

        setTimeout(function() {
        }, 10);

        // Clean up DOM here
  • Before and after each test the DOM is mocked an cleaned.
  • In the 'bleep bloop should async DOM manipulate' test, the asyncBleep function will output 'Hello' to the DOM in the next animation frame.
  • A function is available in the first argument (called 'done' in the above example) as a callback.
  • When this argument is used, Mocha knows to wait until the callback is called before resolving the test.
  • It's that argument that allows this unit test to run without errors.

The pattern in the above example is simple, easy to write and remember, and should suffice in even the most complex DOM manipulation scenarios.