How do I resolve the following promises?

Question

I'm trying to resolve the promise in the getABC function, but the local const variable b returns undefined. How do I implement the callback function used by the getB function so that it returns the destructured array item with the value of 'B'.

const [A, B, C] = ['A', 'B', 'C'];

function getA() {
  return A;
}

function getB(callback) {
  setTimeout(() => {
    callback(B);
  }, 10);
}

function getC() {
  return Promise.resolve().then(() => C);
}

function getABC() {
  return new Promise((resolve, reject) => {
    const a = getA();
    const b = getB(b => b);
    const c = getC().then(item => item).catch(err => console.log(err));

    resolve([a, b, c])
  });
}

getABC().then((arr) => console.log(arr));

Your `getB` doesn't return anything. You just give it a callback that essentially does nothing. It just gets called 10 milliseconds later. Was it meant to actually resolve a promise 10ms later? — VLAZ, Jan 03 '22 at 18:48
`function getABC() { return Promise.all([getA(), new Promise(getB), getC()]); }` But really you should just change the implementation of `getB` to return a promise instead of accepting a callback. — Bergi, Jan 03 '22 at 18:56
@MoonShine - "_I'm trying to resolve the promise in_" & "_How do I implement the callback function_" - it _appears like_ you wrote it based on your question - regardless, Bergi answered the question you wrote very directly. — Randy Casburn, Jan 03 '22 at 19:12

konsolebox · Answer 1 · 2022-01-03T19:05:30.323

You also have to wrap setTimeout inside a Promise, and call Promise.all before resolve:

const [A, B, C] = ['A', 'B', 'C'];

function getA() {
  return A;
}

function getB(callback) {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      resolve(callback(B));
    }, 10);
  });
}

function getC() {
  return Promise.resolve().then(() => C);
}

function getABC() {
  return new Promise((resolve, reject) => {
    const a = getA();
    const b = getB(b => b);
    const c = getC().then(item => item).catch(err => console.log(err));

    resolve(Promise.all([a, b, c]));
  });
}

getABC().then(arr => console.log(arr));

All of these can truly be simplified if you use async/await.

How do I resolve the following promises?

1 Answers1