Error is not thrown when running inside Mocha test

[xixixao]

Ok, this is a really strange bug and quite likely might belong to Mocha. Given that I likely won't get help there, if anyone has any useful input, I will be grateful.

Promise = require 'bluebird'

describe 'test', ->
  it 'should throw an error', (done) ->
    Promise.delay 10
    .done ->
      throw new Error "bla"

This test times out. Note that all of the below simply thrown an error:

describe 'test', ->
  it 'does throw an error', (done) ->
    setTimeout ->
      throw new Error "bla"
    , 10
describe 'test', ->
  it 'does throw an error', (done) ->
    throw new Error "bla"

(this has nothing to do with time, I am just using it for an example). Thanks!

[petkaantonov]

You are not returning a promise from the test and you are not calling the done argument passed to your test so mocha treats it as synchronous test but errors thrown in done are thrown in the process asynchronously

[jamiebuilds]

@petkaantonov Coffeescript automatically adds the return. So that code example expands to:

var Promise;

Promise = require('bluebird');

describe('test', function() {
  return it('should throw an error', function(done) {
    return Promise.delay(10).done(function() {
      throw new Error("bla");
    });
  });
});

I've also tested Async errors without calling done() which works fine:

it('should throw Error', function(done) {
  throw new Error('Exit');
});

it('should throw Error inside setTimeout', function(done) {
  setTimeout(function() {
    throw new Error('Exit');
  }, 200);
});

The errors are reproducible without the done argument:

it('should throw Error inside promise', function() {
  return Promise.delay(10).done(function() {
    throw new Error('Exit');
  });
});
// >> No Error is thrown to Mocha

Although it appears to be a problem with Bluebirds done method, finally works:

it('should throw Error inside promise', function() {
  return Promise.delay(10).finally(function() {
    throw new Error('Exit');
  });
});
// >> Error is thrown

[tgriesser]

.done doesn't return a promise, it returns undefined... so as @petkaantonov said, because you're not returning a promise, Mocha assumes it's sync - but it's not, you're throwing async.

Also worth noting, there's typically no need for using .done with bluebird as it's smart enough to log unhandled exceptions.

[domenic]

Also promise-returning tests should use () not (done) in the header

[jamiebuilds]

@tgriesser I did not know .done didn't return a Promise. @xixixao This works fine:

it('should throw Error', function(done) {
  return Promise.delay(10).done(function(text) {
    throw new Error(text);
    done();
  });
});
// >> throws Error

[domenic]

@thejameskyle Right because you are not using Mocha's promise-based async testing, but instead its done(). The two are incompatible.

[xixixao]

@petkaantonov I don't think Mocha treats this as a synchronous test. If it did, the test wouldn't time out. Mocha only looks at the it callback, sees it takes an argument and hence waits for it to be called. See the setTimeout example.

@thejameskyle That code is equivalent to mine and indeed runs, but I am on a different machine atm. I'll have to retest on mine again tonight. Closing for now until I can reproduce it again.

[domenic]

Now that I am off my iPad and on something with a real keyboard: this is the best version IMO.

it('should throw Error', function() { // NOTE: no done
  return Promise.delay(10).then(function(text) { // NOTE: then, not done
    throw new Error(text);
  });
});

[benjamingr]

Not to be a drag, but although it's different (error created at different time) I'd do:

it('should throw Error', function() { // NOTE: no done
  return Promise.delay(10).throw(new Error(text));
});