Callback after all asynchronous forEach callbacks are completed

Question

As the title suggests. How do I do this?

I want to call whenAllDone() after the forEach-loop has gone through each element and done some asynchronous p

Answer 1

It's odd how many incorrect answers has been given to asynchronous case! It can be simply shown that checking index does not provide expected behavior:

// INCORRECT
var list = [4000, 2000];
list.forEach(function(l, index) {
    console.log(l + ' started ...');
    setTimeout(function() {
        console.log(index + ': ' + l);
    }, l);
});

output:

4000 started
2000 started
1: 2000
0: 4000

If we check for index === array.length - 1, callback will be called upon completion of first iteration, whilst first element is still pending!

To solve this problem without using external libraries such as async, I think your best bet is to save length of list and decrement if after each iteration. Since there's just one thread we're sure there no chance of race condition.

var list = [4000, 2000];
var counter = list.length;
list.forEach(function(l, index) {
    console.log(l + ' started ...');
    setTimeout(function() {
        console.log(index + ': ' + l);
        counter -= 1;
        if ( counter === 0)
            // call your callback here
    }, l);
});