Coding Challenge Practice - Question 91

The task is to implement the async function helper parallel()

The boilerplate code

function parallel(funcs){
  // your code here
}

The parallel function is supposed to accept an array of async functions, run all the functions at the same time, with each function having the form func(callback).

If there are no functions

if (funcs.length === 0) {
  callback(undefined, []);
  return;
}

Store the outputs by index, count the finished tasks and ensure the callback fires only once

const results = [];
let completed = 0;
let hasError = false;

To run all the functions in parallel

funcs.forEach((func, index) => {
  func((error, result) => {

To prevent multiple callbacks

if (hasError) return;

To handle errors

if (error) {
  hasError = true;
  callback(error, undefined);
  return;
}

To preserve the order even if tasks finish out of order

results[index] = result;

The callback should be called when all the tasks are completed

completed++;

if (completed === funcs.length) {
  callback(undefined, results);
}

The final code

function parallel(funcs){
  // your code here
  return function (callback) {
    if(funcs.length === 0) {
      callback(undefined, []);
      return;
    }

    const results = [];
    let completed = 0;
    let hasError = false;

    funcs.forEach((func, index) => {
      func((error, result) => {
        if(hasError) return;

        if(error) {
          hasError = true;
          callback(error, undefined);
          return;
        }
        results[index] = result;
        completed++;

        if (completed === funcs.length) {
          callback(undefined, results)
        }
      })
    })
  }
}

That's all folks!