How to expect an asynchronously thrown exception i

Question

Given an Angular2/Typescript method that returns nothing and implements a .subscribe() handler which might throw, such as this:

onSubmit() { // returns nothing
  this.service.someCall(this.someData).subscribe(
    data => {
      return Promise.reject('This is an asynchronously thrown error.');
    },
    err => {},
  );
}

(For the moment, let's assume that there's a good reason for this .subscribe() handler to (probably conditionally) reject without other testable side-effects, thus resulting only in an error message bubbling up to the top of the application.)

How would one go about testing that this method resulted in a rejected Promise?

I found some people with the same question, but no elegant answers:

How to deal with thrown errors in async code with Jasmine?

https://github.com/jasmine/jasmine/issues/529

https://gist.github.com/badsyntax/7769526

Answer 1

I solved this problem by stubbing the console.error() method (in this case, I am using Sinon):

it('should throw exception from component on submit', (done) => {
  // Jasmine isn't capable of capturing an exception thrown from an *asynchronous* operation.
  // However, that error eventually finds its way to console.error(), so we can stub
  // that method and wait for it to be called.
  sandbox.stub(console, 'error').callsFake((...err) => {
    expect(err[1]).toEqual('This is an asynchronously thrown error.');
    done();
  });
  component.onSubmit();
});

Are there any other/better ways to go about this?