Question 250174 in Javascript Question posted in
A very good W3school tutorial can be found here.

Javascript – Adding a Promise to Promise.all() after they all finished

Assuming I have a

function(){
  ....
  return Promise.all([p1,p2])
}

And I want to instead return a promise of p1, p2, p3, with p3 starting only after p1 and p2 are done. What would be the best way?

Would the following work:

function(){
  ....
  return Promise.all([p1,p2]).then(
    (p) => {
      return Promise.all(p.push(p3));
    })
}

Anything special to handle rejected promises?

Tags:

2

Answers


  1. 0

    What you had would work conceptually, but not practically because p.push() returns the new length of the array, not the array. So, you could fix it like this:

    function(){
  ....
  return Promise.all([p1,p2]).then((values) => {
      values.push(fn3());  // start operation 3 and add p3 to the array
      return Promise.all(values);
  });
}

    This also assumes that you only start the operation that returns p3 after the first Promise.all() returns.

    I would personally prefer to use await like this, but it gives the same result:

    async function someFunc() {
    ...
    const values = await Promise.all([p1, p2]);
    values.push(fn3());  // start operation 3 and add p3 to the array
    return Promise.all(values);
}

    This works because Promise.all() accepts an array of promises or values or a mix of the two. For values in the array (anything other than a promise), it just immediately adds them to the results array (no need to wait for a promise).

    Or, perhaps a little simpler to read where the code is a bit more self descriptive for what you actually want to do, you could just do this:

    async function someFunc() {
    ...
    const values = await Promise.all([p1, p2]);
    values.push(await fn3());   // start operation 3
    return values;
}

    This last option would be my favorite.

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  2. 0

    Spread syntax

    If you prefer the format you’ve already got, you could alternatively use the array spread syntax to do this:

    function(){
  ....
  return Promise.all([p1,p2]).then(
    (p) => {
      const p3 = f3(); // start p3
      return Promise.all([...p, p3]);
    })
}

    Error handling

    Take note that the documentation for Promise.all states:

    It rejects when any of the input’s promises rejects, with this first rejection reason.

    So whichever promise rejects first will trigger a reject for Promise.all() and only that rejection is a provided as an argument for catch((err) => { ... }).

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