Promise.any Polyfill

It is common during interviews for interviewers to ask questions on promises like implementing polyfill for Promise.any() as they are one of the trickiest things to understand.

According to MDN –

Promise.any() takes an iterable of Promise objects. It returns a single promise that fulfills as soon as any of the promises in the iterable fulfills, with the value of the fulfilled promise. If no promises in the iterable fulfill (if all of the given promises are rejected), then the returned promise is rejected with an AggregateError, a new subclass of Error that groups together individual errors.

In simple terms Promise.any() is just opposite of Promise.all().

Polyfill for Promise.any().

Reading the definition we can break the problem statement into multiple sub-problems and then tackle them individually to implement the polyfill.

• Function takes an array of promises as input and returns a new promise.
• The returned promise is resolved as soon as any of the input promises resolves.
• Else if all of the input promises are rejected then the returned promise is rejected with the array of all the input promises reasons.

const any = function(promisesArray) {
  const promiseErrors = new Array(promisesArray.length);
  let counter = 0;
  
  //return a new promise
  return new Promise((resolve, reject) => {
    promisesArray.forEach((promise, index) => {
      Promise.resolve(promise)
        .then(resolve) // resolve, when any of the input promise resolves
        .catch((error) => {
          promiseErrors[index] = error;
          counter = counter + 1;
          if (counter === promisesArray.length) {
            // all promises rejected, reject outer promise
            reject(promiseErrors);
          }
      }); // reject, when any of the input promise rejects
    });
  });
};

Test case 1

Input:
const test1 = new Promise(function (resolve, reject) {
  setTimeout(reject, 500, 'one');
});

const test2 = new Promise(function (resolve, reject) {
  setTimeout(resolve, 600, 'two');
});

const test3 = new Promise(function (resolve, reject) {
  setTimeout(reject, 200, 'three');
});

any([test1, test2, test3]).then(function (value) {
  // first and third fails, 2nd resolves
  console.log(value);
}).catch(function (err){
  console.log(err);
});

Output:
"two"

Test case 1

Input:
const test1 = new Promise(function (resolve, reject) {
  setTimeout(reject, 500, 'one');
});

const test2 = new Promise(function (resolve, reject) {
  setTimeout(reject, 600, 'two');
});

const test3 = new Promise(function (resolve, reject) {
  setTimeout(reject, 200, 'three');
});

any([test1, test2, test3]).then(function (value) {
  console.log(value);
}).catch(function (err){
  // all three fails
  console.log(err);
});

Output:
["one","two","three"]