Execute a collection of tasks in parallel and wait for all of them to finish before executing the callback

gistfile1.txt

function doAll(collection, callback) {
    var left = collection.length;
    collection.forEach(function(fun) {
        fun(function() {
            if (--left == 0) callback();
        });
    });
};

var result = [];

doAll([
    function(callback) {
        setTimeout(function() {result.push(1); callback();}, 2000)
    },
    function(callback) {
        setTimeout(function() {result.push(2); callback();}, 3000)
    },
    function(callback) {
        setTimeout(function() {result.push(3); callback();}, 1000)
    }
], function() { return result; } 
);

// returns [3, 1, 2]