Chained promises broken when testing due to $exceptionHandler interference

[Thorn1089]

The Promises specification has the following to say about chaining promises:

then must return a promise

promise2 = promise1.then(onFulfilled, onRejected);

If either onFulfilled or onRejected returns a value that is not a promise, promise2 must be fulfilled with that value.

If either onFulfilled or onRejected throws an exception, promise2 must be rejected with the thrown exception as the reason.

However, when using Karma to test a module, I noticed that the following inside the method under test failed:

return $http.post(endpoint, data).then(function(response) {
  //return some data from the response
}, function(error) {
  var throwable;
  //grab some data from the response to create a meaningful rejection reason
  throw throwable;
});

When invoking the method as follows, with a mocked HTTP backend to force a failure:

service.do(args).then(function(unused) {
  expect(false).toBe(true);//In the absence of a fail() method, use a contradiction
}, function(error) {
  expect(error.code).toBe(FAILURE_VAL);
});

Instead of the expectation being met in the onRejected function, the exception propagates up the stack and fails the test. This is because apparently during tests, the default policy of $exceptionHandler is rethrow.

I don't understand what business $exceptionHandler has at all being invoked by $q, especially since this behavior violates the Promises/A specification. Exceptions being thrown is a normal and expected part of promise execution.

[FrigoEU]

I'm wondering if this has been looked at in the meantime? I'm having the same problem where my code is correctly throwing an error to reject a promise and then handle it in my .catch() handler, but angular is intercepting the error and immediately propagating it upward making karma think there was an uncaught error.

[rasmusvhansen]

+1

I also find this behaviour odd. I guess a workaround is to configure the exceptionhandler to log instead of rethrow exception.

I use this in the relevant tests:

beforeEach(module(function ($exceptionHandlerProvider) {
  $exceptionHandlerProvider.mode('log'); // See https://github.com/angular/angular.js/issues/3174
}));

[Narretz]

$q is not promises/A compliant, IIRC. But it's definitely something to keep in mind for a future rewrite @caitp

[caitp]

It is actually aplus compliant, at least a few months ago it was, not sure how much aplus has changed since then =) Work is being done on refactoring the logging of exceptions (ITF: rejections in general), however @rasmusvhansen's solution is really the best you can do right now, short of monkey-patching ngMock

[n-rook]

Hey @alexeagle, how are you doing? You still like testing right? That means you should fix this bug ;)

Here's a jsfiddle demonstrating it: http://jsfiddle.net/tz8f7tvs/2/

[alexeagle]

Hi Nate, I do like testing. But I don't know anything about Angular 1. Do you want to spend an "escalation token" on this one? :)

[n-rook]

Oh, I didn't realize the teams were split, too! I saw Angular 2 just launched publicly, so congratulations on that.

As a Xoogler, I'm not sure I get escalation tokens 😳 If I do, though, I'd love to see this fixed, since I think it makes it really hard to write good tests in some cases.