Promise.all polyfill

Problem Statement

TLDR:: Implement a function which mimcks behaviour of Promise.all()

All Browsers don’t support Promise.all() method, this can lead to compatibility issue and inconsistent behaviour across different browsers. Hence we need to create a polyfill that can provide the same functionality as Promise.all for all browsers that don’t support it natively;

Task here is to create the polyfill that should be able to handle all of the features and functionality of Promise.all(), including the ability to handle an array of promises and return a single promise that resolves when all of the promises in the array have resolved, or rejects if any of the promises reject.

Approach

To implement this we need to understand how Promise.all works, without going into too much discussion, I drilled down the requirement for us below:

Should return all resolved results in promises in an array once all promises are completed;
If any one of it fails, it should cancel and directly return the error object;
By default, I checked this and if only resolve promise without any response, it returns array of undefined.

Implementation

Promise.myAll = function (promises) {
  let completedPromises = promises.length;
  let results = new Array(promises.length).fill(undefined);

  return new Promise((resolve, reject) => {
    // if all the promises are completed, 
    // the checkIfDone will resolve and return the results
    let checkIfDone = () => {
      if (--completedPromises === 0) resolve(results);
    };

    promises.forEach((promise, index) => {
      promise
        .then((response) => {
          results[index] = response;
        })
        .catch((error) => {
          reject(error);
        })
        .then(checkIfDone);
    });
  });
};

Example Test -

// Input:
function dummyPromise(time) {
  return new Promise(function (resolve, reject) {
    setTimeout(function () {
      resolve(time);
    }, time);
  });
}

const dummyTaskList = [
  dummyPromise(1000), 
  dummyPromise(2000), 
  dummyPromise(5000)
];

// Run Promise.myAll
Promise.myAll(dummyTaskList).then(
  (response) => {
    console.log(response);
  },
  (error) => console.log(error)
);

// Output:
// >> [ 1000, 2000, 5000 ]

Example Test using Promise which throws error -