Questions regarding unhandled rejections about promise HOT 4 CLOSED

False positive:

const promiseThatResolvesSlowly = delay(1000);
const promiseThatRejectsQuickly = Promise.reject('oops');

try {
  await promiseThatResolvesSlowly;
  await promiseThatRejectsQuickly;
} catch (ex) {
  // We handle the rejection here, but it was already reported as an
  // unhandled rejection while we were waiting for promiseThatResolvesSlowly
}

The reason it's not recommended in production, is that promise must track all unhandled rejections in case they are later resolved and we need to report them as handled. This has a memory overhead and can cause a memory leak in applications that deliberately ignore some rejected promises.

from promise.

Thanks for the explanation!

You probably meant for that to run in parallel?

const promiseThatResolvesSlowly = delay(1000);
const promiseThatRejectsQuickly = Promise.reject('oops');

try {
  await Promise.all([
    promiseThatResolvesSlowly, 
    promiseThatRejectsQuickly,
  ]);
} catch (ex) {
  // We handle the rejection here, but it was already reported as an
  // unhandled rejection while we were waiting for promiseThatResolvesSlowly
}

I think I'm starting to understand the problem better now, after a year of React experience 😄

I'm wondering why the best practices here are not popularized more, instead of just being hinted at in this obscure issue: facebook/react-native#2585

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...my bad, your example is correct, mine isn't (Promise.all only waits for first rejection)

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@alamothe more importantly, Promise.all would mark the rejection as handled whenever it happens. so even if you had multiple rejections, all with different delays, they would all be marked as handled. Only the first would actually be re-thrown into the try/catch block though.

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