PromiseAll-Polyfill.js

/* 💡"JavaScript-with-JC"
👉 Promise.all() and Its Polyfill
Promise.all() - It executes all passed promises concurrently and improves the performance of the application.

💡Promise.all() Cases :-
1) If all promises resolve, returns the array of results of all promises resolved.
2) If any promise fails, returns the rejected promise error.
3) If passed empty [], returns empty [].
*/

const p1 = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve("1st Promise resolved!");
  }, 1000);
});

const p2 = Promise.resolve("2nd Promise resolved!");

const p3 = 3;

const p4 = new Promise((resolve, reject) => {
  setTimeout(() => {
    let status = true;
    if (status) {
      resolve("4th Promise resolved!");
    } else {
      reject("4th Promise rejected!");
    }
  }, 2000);
});

Promise.all([p1, p2, p3, p4])
  .then((result) => {
    console.log("result all", result);
  })
  .catch((error) => {
    console.log("error all", error);
  });

//💡Polyfill of Promise.all
Promise.customAll = function (promisesArray) {
  // 👇 return a new promise
  return new Promise((resolve, reject) => {
    const result = [];

    // 👇 to check how many promises are completed
    let completed = 0;

    // 👇 if passed as empty [] then return empty []
    if (promisesArray.length === 0) {
      resolve(result);
    }

    // 👇 execute each promise of promisesArray
    for (let i = 0; i < promisesArray.length; i++) {
      Promise.resolve(promisesArray[i])
        .then((response) => {
          // 👇 if promise passes store its response and increment the count
          result[i] = response;
          completed++;

          // 👇 if all the promises are completed,
          //resolve and return the result array
          if (completed === promisesArray.length) {
            resolve(result);
          }
        })
        .catch((error) => {
          // 👇 if any promise fails, reject.
          reject(error);
        });
    }
  });
};

Promise.customAll([p1, p2, p3, p4])
  .then((result) => {
    console.log("result customAll", result);
  })
  .catch((error) => {
    console.log("error customAll", error);
  });

/* output :-
 [
   "1st Promise resolved!",
   "2nd Promise resolved!",
   3,
  "4th Promise resolved!",
 ]; */