Unhandled promise rejection: false positive

With karma 6.3.3, I’m getting an Unhandled promise rejection error, which looks like a false positive to me. The same code worked fine with karma 4.4.1 (see commit before latest).

The problem can be reproduced as follows:

git clone https://github.com/andreashuber69/karma-false-positive-repro.git
cd karma-false-positive-repro
npm ci
npm run test

[devoto13]

Actually this has nothing to do with updating karma. You can reproduce this issue by only updating karma-jasmine, which in turn would update jasmine-core to the version, which includes https://github.com/jasmine/jasmine/pull/1778. So you should probably track https://github.com/jasmine/jasmine/issues/1843, which also shows a workaround for the use cases where you need to trigger unhandled rejection. Unless you specifically testing the logic around error handling, it may be better to re-structure your test in a way that avoids unhandled promise rejection in the first place.

Having said that I would like to see if we can solve SourceMap position not found for trace warnings produced by karma somehow. You see them because TypeScript has down-leveled the async/await syntax and some of the frames point to the locations in this down-leveled code, which don’t map to anything in the original TS code, so that’s why the warning. Maybe it would be better to drop such frames from the re-mapped stack trace instead.

[devoto13]

@andreashuber69 AFAIU the rejection detection logic works synchronously, e.g. if you don’t await the rejected Promise synchronously, then it will be considered unhandled. In your test case, you have await new Promise<void>((resolve) => setTimeout(resolve, 100));, so you create a rejected (not awaited) Promise in one tick, before that line, and you await it in another tick after that line when it’s too late. If you move const rejectingPromise = Promise.reject(new Error("Operation failed.")); to after the timeout, then everything should work correctly.