2721. Execute Asynchronous Functions in Parallel

In this challenge, you must implement a function that accepts an array of asynchronous functions and executes them all in parallel, returning a single Promise.

📄 Problem Statement

Given an array of functions functions, each returning a Promise, return a new Promise that:

Resolves when all Promises resolve successfully. The resolved value must be an array of their results in order.
Rejects immediately if any Promise rejects, with the reason of the first rejection.

🚫 Do not use Promise.all.

💡 Example


const asyncFunc1 = () => new Promise(res => setTimeout(() => res("A"), 100));
const asyncFunc2 = () => new Promise(res => setTimeout(() => res("B"), 50));

executeFunctionsInParallel([asyncFunc1, asyncFunc2])
  .then(console.log); // ["A", "B"]

🧠 Explanation

You manually track the results and completions of each Promise. You also monitor for rejection. Once all promises are resolved, the main Promise resolves with the ordered results.

✅ Your Solution


/**
 * @param {Array<() => Promise>} functions
 * @return {Promise}
 */
var executeFunctionsInParallel = function(functions) {
    return new Promise((resolve, reject) => {
        const results = [];
        let completed = 0;
        let hasError = false;

        functions.forEach((fn, i) => {
            fn()
                .then((val) => {
                    if (hasError) return;
                    results[i] = val;
                    completed++;
                    if (completed === functions.length) {
                        resolve(results);
                    }
                })
                .catch((err) => {
                    if (!hasError) {
                        hasError = true;
                        reject(err);
                    }
                });
        });
    });
};