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!