AlgoDaily - Implement Promise.all - One Pager Cheat Sheet

One Pager Cheat Sheet

Promise.all() is used to take an iterable of promises as an input, wait for them to resolve, and return a single Promise that resolves to an array of the results, but will reject with the first rejection message or error if any of the input promises are rejected.
We can implement an all function that takes an array of promises and returns a Promise that resolves to an array of return values from the input.

This is our final solution.

To visualize the solution and step through the below code, click Visualize the Solution on the right-side menu or the VISUALIZE button in Interactive Mode.

xxxxxxxxxx
 
Promise.all = (promises) => {
  return new Promise((resolve, reject) => {
    const results = [];
​
    if (!promises.length) {
      resolve(results);
      return;
    }
​
    let pending = promises.length;
​
    promises.forEach((promise, idx) => {
      Promise.resolve(promise).then((value) => {
        results[idx] = value;
        pending--;
        if (pending === 0) {
          resolve(results);
        }
      }, reject);
    });
  });
};

Alright, well done! Try another walk-through.

If you had any problems with this tutorial, check out the main forum thread here.