@jbedard, thanks for your PR! By analyzing the history of the files in this pull request, we identified @gibson042, @jaubourg and @markelog to be potential reviewers.

No tests? There is also code-style issue

I wasn't really sure how this could be unit tested. Any ideas? With everything hidden in closures it's a little hard to do things like spy on the callback methods...

I'm not sure why we need both callbacks and .then handlers. Could we instead merge those Callbacks? EDIT: I think it's because error handling works different?

Yes, among other reasons. .done callbacks fire synchronously and can lock up, but the first thing they do is kick off asynchronous .then callbacks (which can't).

@gibson042 I've moved everything into the existing list.add call.

Interesting... I amended the commit just to add the PR number to the commit msg and one test started failing. Now I ran it again and everything passed. Anyone ever see something similar before? I can't seem to reproduce it locally.

Here's the travis job that failed: https://travis-ci.org/jquery/jquery/jobs/242269140

The test failure:

  1) 2.2.6: `then` may be called multiple times on the same promise. 2.2.6.2: If/when `promise` is rejected, all respective `onRejected` callbacks must execute in the order of their originating calls to `then`. `onRejected` handlers are called in the original order even when one handler is added inside another handler immediately-rejected:
     Uncaught AssertError: expected handler1, handler2, handler3 to be called in order but were called as handler1, handler2
      at Object.fail (node_modules/sinon/lib/sinon/assert.js:90:29)
      at failAssertion (node_modules/sinon/lib/sinon/assert.js:51:24)
      at Object.assertCallOrder [as callOrder] (node_modules/sinon/lib/sinon/assert.js:118:21)
      at null._onTimeout (node_modules/promises-aplus-tests/lib/tests/2.2.6.js:249:42)