How to Test Async Code with Mocha

Jun 14, 2019

The Mocha test framework has excellent support for async tests. Generally speaking, there are 3 ways to structure async tests with Mocha:

In this tutorial, you'll learn how to write Mocha tests using each of these 3 paradigms.

Setup

Suppose you have an asynchronous function that makes an HTTP request using Axios.

const axios = require('axios');

function get(url, cb) {
  return axios.get(url);
}

The function get() takes one parameter, a URL, and returns a promise. Let's test this function with Mocha, 3 different ways.

Async Functions

Mocha supports async functions out of the box, no plugins or configuration needed. You can pass an async function to it(), and Mocha will handle any errors that occur.

describe('get()', function() {
  it('works', async function() {
    const res = await get('http://httpbin.org/get?answer=42');
    assert.equal(res.data.args.answer, 42);
  });
});

Promises

You might be surprised to know that Mocha has supported async functions since 2014. Async functions were only introduced in 2017, but async functions return promises, and Mocha has supported promises since before they were formally introduced into JavaScript.

In other words, if you return a promise or promise from your it() function, Mocha will handle it for you.

describe('get()', function() {
  it('works', function() {
    return get('http://httpbin.org/get?answer=42').
      then(res => assert.equal(res.data.args.answer, 42));
  });
});

Callbacks using done()

In older codebases, you may see callback-based tests using Mocha's done() function. This style is a bit archaic since promises have been a part of JavaScript since 2015 and async/await since 2017, but you may run into it at some point.

Mocha inspects the function you pass to it(). If that function takes a parameter, Mocha assumes that parameter is a done() callback that you will call to indicate your test is done. Here's how you would test the get() function using Mocha's done() callback.

describe('get()', function() {
  it('works', function(done) {
    get('http://httpbin.org/get?answer=42').
      then(res => {
        assert.equal(res.data.args.answer, 42);
        // `done()` with no parameters means the test succeeded
        done();
      }).
      // If you pass a parameter to `done()`, Mocha considers that an error
      catch(err => done(err));
  });
});

If you call done() with no arguments, you're telling Mocha that your test succeeded. If you pass a parameter, Mocha assumes that parameter is an error.

Make sure you call done()! If you don't call done(), your Mocha test will time out. You'll see the below error:

Error: Timeout of 2000ms exceeded. For async tests and hooks, ensure "done()" is called

If you see this error, either you need to increase your Mocha test timeout or there's some bug in your code that's preventing done() from getting called.


More Mocha Tutorials