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.

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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);

    });

  });

};

That's all we've got! Let's move on to the next tutorial.

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