Reject a promise from then()

Question

How can you reject a promise from inside its then()?

For example:

Promise.all(promiseArr).then(()=>{
  if(cond){
    //reject
  }
}).catch(()=>{ /*do something*/ });

The only relevant question I found was this: How to reject a promise from inside then function but it's from 2014, so there must be a better way then to throw by now with support of ES6.

A rejected promise is just the async way to describe an Error — Thomas, Apr 09 '17 at 12:28
ideally, something that describes the error that happened any you'll want to show in your console if unhandled. — Thomas, Apr 09 '17 at 12:37
`throw`ing is still perfectly valid in ES6. You could also `return Promise.reject()` if `throw` feels unnatural. — CodingIntrigue, Apr 09 '17 at 12:45

score 11 · Accepted Answer · answered Apr 09 '17 at 16:07

11

ES6/ES2015 is still JavaScript and doesn't offer anything new regarding promise rejection. In fact, native promises are ES6.

It is either

promise
.then(() => {
  return Promise.reject(...);
})
.catch(...);

or

promise
.then(() => {
  throw ...;
})
.catch(...);

And throw is more idiomatic (and generally more performant) way to do this.

This may not be true for other promise implementations. E.g. in AngularJS throw and $q.reject() is not the same thing.

answered Apr 09 '17 at 16:07

Estus Flask

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"E.g. in AngularJS throw and $q.reject() is not the same thing.": can you elaborate on this or provide a link to a more detailed information? I can't find anything regarding those differences. – ClementParis016 Nov 03 '17 at 17:31
I'm not sure if this is described in AngularJS docs, but `throw` inside `catch` or `then` results in uncaught error, while `return $q.reject()` results in rejected promise. – Estus Flask Nov 03 '17 at 18:19

score 2 · Answer 2 · answered Jul 10 '18 at 18:20

I have found out (after hours of scratching my head) that if a throw is within an async method such as setTimeout(() => throw new Error('description')) it will be treated as an Unhandled Exception and will NOT end up in your .catch() block (you'll only see it in the console window) - see this article where I found out about that difference between throw and Promise.reject.

Reject a promise from then()

2 Answers2

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