Getting a UnhandledPromiseRejectionWarning when testing using mocha/chai

Question

So, I'm testing a component that relies on an event-emitter. To do so I came up with a solution using Promises with Mocha+Chai:

it('should transition with the correct event', (done) => {
  const cFSM = new CharacterFSM({}, emitter, transitions);
  let timeout = null;
  let resolved = false;
  new Promise((resolve, reject) => {
    emitter.once('action', resolve);
    emitter.emit('done', {});
    timeout = setTimeout(() => {
      if (!resolved) {
        reject('Timedout!');
      }
      clearTimeout(timeout);
    }, 100);
  }).then((state) => {
    resolved = true;
    assert(state.action === 'DONE', 'should change state');
    done();
  }).catch((error) => {
    assert.isNotOk(error,'Promise error');
    done();
  });
});

On the console I'm getting an 'UnhandledPromiseRejectionWarning' even though the reject function is getting called since it instantly shows the message 'AssertionError: Promise error'

(node:25754) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 2): AssertionError: Promise error: expected { Object (message, showDiff, ...) } to be falsy 1) should transition with the correct event

And then, after 2 sec I get

Error: timeout of 2000ms exceeded. Ensure the done() callback is being called in this test.

Which is even weirder since the catch callback was executed.(I think that for some reason the assert failure prevented the rest of the execution)

Now the funny thing, if I comment out the assert.isNotOk(error...) the test runs fine with out any warning in the console. It stills 'fails' in the sense that it executes the catch.
But still, I can't understand these errors with promise. Can someone enlighten me?

Answer 1

The issue is caused by this:

.catch((error) => {
  assert.isNotOk(error,'Promise error');
  done();
});

If the assertion fails, it will throw an error. This error will cause done() never to get called, because the code errored out before it. That's what causes the timeout.

The "Unhandled promise rejection" is also caused by the failed assertion, because if an error is thrown in a catch() handler, and there isn't a subsequent catch() handler, the error will get swallowed (as explained in this article). The UnhandledPromiseRejectionWarning warning is alerting you to this fact.

In general, if you want to test promise-based code in Mocha, you should rely on the fact that Mocha itself can handle promises already. You shouldn't use done(), but instead, return a promise from your test. Mocha will then catch any errors itself.

Like this:

it('should transition with the correct event', () => {
  ...
  return new Promise((resolve, reject) => {
    ...
  }).then((state) => {
    assert(state.action === 'DONE', 'should change state');
  })
  .catch((error) => {
    assert.isNotOk(error,'Promise error');
  });
});