2721-execute-asynchronous-functions-in-parallel.js

/**
 * 2721. Execute Asynchronous Functions in Parallel
 * https://leetcode.com/problems/execute-asynchronous-functions-in-parallel/
 * Difficulty: Medium
 *
 * Given an array of asynchronous functions functions, return a new promise promise.
 * Each function in the array accepts no arguments and returns a promise. All the
 * promises should be executed in parallel.
 *
 * promise resolves:
 * - When all the promises returned from functions were resolved successfully in
 * parallel. The resolved value of promise should be an array of all the resolved
 * values of promises in the same order as they were in the functions. The promise
 * should resolve when all the asynchronous functions in the array have completed
 * execution in parallel.
 *
 * promise rejects:
 * When any of the promises returned from functions were rejected. promise should
 * also reject with the reason of the first rejection.
 *
 * Please solve it without using the built-in Promise.all function.
 */

/**
 * @param {Array<Function>} functions
 * @return {Promise<any>}
 */
var promiseAll = function(functions) {
  return new Promise((resolve, reject) => {
    const promises = new Array(functions.length);
    let count = 0;
    functions.forEach((fn, i) => {
      fn().then(result => {
        promises[i] = result;
        if (++count === promises.length) {
          resolve(promises);
        }
      }).catch(reject);
    });
  });
};