Rejecting a Promise with another new Promise

Question

Im playing around with promises and i wanted a way of rejecting the promise inside of a then callback. So this is done by either calling throw return or return Promise.reject();. So far so good. You can accomplish this by also calling new Promise.reject(); withou a return.

Can someone explain why does this work?

new Promise((res, rej) => {
  setTimeout(() => {
    res(200);
  }, 2000);
})
  .then(() => {
    console.log("ok1");
    new Promise.reject();
    // return Promise.reject(); 
  .then(() => {
    console.log("ok2");
  })
  .catch(() => {
    console.log("error");
  });

Answer 1

 new Promise.reject()

Your code throws an exception. Exceptions cause promises to be rejected.

Answer 2

Because Promise.reject is not a constructor, this code works fine:

new Promise((res, rej) => {
  setTimeout(() => {
    res(200);
  }, 2000);
})
  .then(() => {
    console.log('ok1')
    Promise.reject('error!')
})
  .then(() => {
    console.log("ok2");
  })
  .catch(() => {
    console.log("error");
  });

Answer 3

New Promise.reject throws an error so the code jumps to the catch section with the following error: Promise.reject is not a constructor

 new Promise((resolve, reject) => { setTimeout(() => { resolve('OK'); }, 2000); }).then((res) => { console.log(res); new Promise.reject(); }).then((res) => { console.log(res); }).catch((err) => { console.log('error',err.message); });

Answer 4

That's because of the inner workings of Promise.reject . It basically does:

Promise.reject = function(err) {
  return new this((_, rej) => rej(err));
};

Now if you call it with new , this refers to a newly created object, and that's not a constructor.

 Promise.reject(); // this == Promise -> constructor
 new Promise.reject(); // this == {} -> not a constructor

Therefore, the .then callback throws an error, and that causes the returned Promise to be rejected with that error.