When Promise state become rejected or resolved

Question

let promise = new Promise(function(resolve, reject) {
      setTimeout(() => reject(new Error("Whoops!")), 1000);
});

// reject runs the second function in .then
promise.then(
  result => alert(result), // doesn't run
  error => alert(error) // shows "Error: Whoops!" after 1 second
);

In the above code snippets even though I am calling reject but promise state is coming as resolved but when I am removing error => alert(error) from promise.then then I am getting promise state as rejected If one is calling reject then promise state should be rejected not resolved am I correct?

Nitish Narang · Answer 1 · 2018-11-14T12:07:30.583

2

You can also handle Promises in "then-catch" fashion like below

let promise = new Promise(function(resolve, reject) {
  setTimeout(() => reject('Error'));
});

promise
  .then(result => console.log('Result ', result)) 
  .catch(error => console.log('This is Error message -', error))

edited Nov 14 '18 at 12:07

answered Nov 14 '18 at 11:49

Nitish Narang

4,124
2
15
22

`then` accepts two arguments - one for success callback and the other for a rejection callback. Using it a valid way of dealing with Promise rejection. – Szab Nov 14 '18 at 12:03
A promise should have exception handling mechanism. That is why we have `.catch`, the control goes to `.catch` when an exception is thrown from the promise body. if you call `resolve` the control goes to the first executor argument of `.then` and if you call `reject`, then it goes to the second argument of `.then`. So I think we should have both `then` and `catch` – Parnab Sanyal Nov 14 '18 at 12:56

Husam Ibrahim · Accepted Answer · 2018-11-14T12:48:28.353

The promise that is resolved is not the original promise, but the promise returned by then. The original promise is indeed rejected, and you can verify that by console.log(promise). But because you chained then it returns another promise ..

Return value [of then]

A Promise in the pending status. The handler function (onFulfilled or onRejected) then gets called asynchronously (as soon as the stack is empty). After the invocation of the handler function, if the handler function:

returns a value, the promise returned by then gets resolved with the returned value as its value;

doesn't return anything, the promise returned by then gets resolved with an undefined value;

throws an error, the promise returned by then gets rejected with the thrown error as its value;

...

This second point is what applies to your case. You can verify that by observing that Promise.reject().then(undefined, ()=>{})
returns a promise that gets resolved with an undefined value.

score 0 · Answer 3 · answered Nov 14 '18 at 11:56

0

The final status of the promise is also rejected in your case. Try console.log(promise); It has nothing to do with error => alert(error)

answered Nov 14 '18 at 11:56

Monica Acha

1,076
9
16

score 0 · Answer 4 · answered Nov 14 '18 at 12:02

The code you posted seems to work correctly both on Chrome and Firefox - the Promise was rejected as expected. reject function marks the Promise as rejected - you can then react upon that rejection either by passing a callback function into a second argument of then or by using the catch method. Both approaches are correct.

If you are encountering any unexpected behaviour, verify that you are not using some faulty polyfill that replaces the original Promise object.

Reference: Promise.prototype.then, Promise.prototype.catch

When Promise state become rejected or resolved

4 Answers4