How to test promises with Mocha

Question

I\'m using Mocha to test an asynchronous function that returns a promise.

What\'s the best way to test that the promise resolves to the correct value?

Answer 1

Then 'returns' a promise which can be used to handle the error. Most libraries support a method called done which will make sure any un-handled errors are thrown.

it('does something asynchronous', function (done) {
  getSomePromise()
    .then(function (value) {
      value.should.equal('foo')
    })
    .done(() => done(), done);
});

You can also use something like mocha-as-promised (there are similar libraries for other test frameworks). If you're running server side:

npm install mocha-as-promised

Then at the start of your script:

require("mocha-as-promised")();

If you're running client side:

Then inside your tests you can just return the promise:

it('does something asynchronous', function () {
  return getSomePromise()
    .then(function (value) {
      value.should.equal('foo')
    });
});

Or in coffee-script (as per your original example)

it 'does something asynchronous', () ->
  getSomePromise().then (value) =>
    value.should.equal 'foo'