Node v6.6 and asynchronously handled promise rejections

Node v6.6.0 added a neat warning when it detects an unhandled promise rejection. For example:

Promise.resolve().then(() => {
  undefinedVariable * 5;
});
// (node:57413) UnhandledPromiseRejectionWarning:
//   Unhandled promise rejection (rejection id: 20): ReferenceError: undefinedVariable is not defined

Prior to this warning, the ReferenceError would get swallowed up and you might be left scratching your head. It was possible to capture these unhandled rejections through other means, but it’s great to have it built-in to Node now.

If you’re seeing this warning on its own, then it’s most likely legitimate and you should try to track it down.

There is, however, a counterpart to the above warning, which is:

PromiseRejectionHandledWarning: Promise rejection was handled asynchronously

This will be raised if you actually do handle the rejection, but in a later tick. This pattern isn’t very common in most applications, but you might encounter it in your test suite.

The culprit for me were some Sinon.JS stubs which were doing this:

sinon.stub(Database, "connect").returns(Promise.reject("nope"));

Although valid - the rejection is raised as part of the test and then handled - it raises the warning because Node has no way to know that it’ll be handled later on in the test. Instead, you’d get the PromiseRejectionHandledWarning warning a few ticks later as an acknowledgement.

The quick fix for me - and something I probably should have done a while ago - was to include the sinon-as-promised module which adds proper resolves() and rejects() functions to stubs. These don’t raise any warnings because the promise rejection is created when the function is actually called.