How can I test that a Promise is never fulfilled?

Question

I'm testing a library I wrote to throttle execution of a function.

The API is throttler.do(fn) and it returns a promise of fn's return value (resolved at whichever point the throttler decides it's ok to run it).

I'm using lolex to fake Date() and setTimeout so if I set the throttler to allow two actions per minute and do

throttler.do(() => {});
throttler.do(() => {});
throttler.do(() => {}).should.eventually.equal(5);

this fails as expected (it times out because it's waiting on the last promise forever since I never call lolex.tick).

Is there a way I can turn this into a passing test, something like

throttler.do(() => {}).should.never.be.fulfilled;

I can't do

setTimeout(() => done(), 1500)

because the setTimeout method is faked by lolex.

score 0 · Accepted Answer · answered Mar 24 '16 at 11:24

0

You can mock the Promise to be synchronous and then check the Promise state. Here is a library you can use promise-sync

answered Mar 24 '16 at 11:24

Slava Shpitalny

Can you elaborate a little bit? I get the idea behind the library but I'm not sure how to apply it. In my case the promise is returned by the subject under test. If I mock what the subject under test returns then I can't really test it. – Mar 24 '16 at 16:01
How does he create the Promise? You can mock the Promise factory, if you use the Q to defer a promise you can mock that – Slava Shpitalny Mar 24 '16 at 16:32
Got it - that's much better than my approach, thanks. – Mar 24 '16 at 16:40

1 Answers1