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.

Answer 1

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.