Promise.all polyfill

Working with promises is not easy until you have a thorough understanding of it works, thus in many interviews we are asked to implement polyfills for Promise.all() method.

As per MDN –

The Promise.all() accepts an array of promises and returns a promise that resolves when all of the promises in the array are fulfilled or when the iterable contains no promises. It rejects with the reason of the first promise that rejects.

Polyfill for Promise.all()

After reading the definition of Promise.all() we can break down the problem in sub-problem and tackle it one by one.

1. It will return a promise.
2. The promise will resolve with result of all the passed promises or reject with the error message of first failed promise.
3. The results are returned in the same order as the promises are in the given array.

function myPromiseAll(taskList) {
  //to store results 
  const results = [];
  
  //to track how many promises have completed
  let promisesCompleted = 0;

  // return new promise
  return new Promise((resolve, reject) => {

    taskList.forEach((promise, index) => {
     //if promise passes
      promise.then((val) => {
        //store its outcome and increment the count 
        results[index] = val;
        promisesCompleted += 1;
        
        //if all the promises are completed, 
        //resolve and return the result
        if (promisesCompleted === taskList.length) {
          resolve(results)
        }
      })
         //if any promise fails, reject.
        .catch(error => {
          reject(error)
        })
    })
  });
}

For testing, you can create a async tasks, using setTimeout and promise which resolves after specified time.

Test case 1.

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

const taskList = [task(1000), task(5000), task(3000)];

//run promise.all
myPromiseAll(taskList)
  .then(results => {
    console.log("got results", results)
  })
  .catch(console.error);

Output:
//"got results" [1000,5000,3000]

Test case 2.

Input:
function task(time) {
  return new Promise(function (resolve, reject) {
    setTimeout(function () {
      if(time < 3000){
        reject("Rejected");
      }else{
        resolve(time);
      }
    }, time);
  });
}

const taskList = [task(1000), task(5000), task(3000)];

//run promise.all
myPromiseAll(taskList)
  .then(results => {
    console.log("got results", results)
  })
  .catch(console.error);

Output:
"Rejected"