When programming Javascript it is sometimes neccessary to start multiple methods, let them run all at the same time and then have a single callback that is called when all of the methods are finished. Dealing with raw Javascript, it is convenient to use a simple object to do the magic for you and just supply methods to add, that is what the class below does:

function AsyncBin(callback) {
 	if (callback===undefined) callback = function() {};
 	
 	this._async = {};
 	this._id = 0;
 	
 	this._callback = callback;
 	this._done = false;
}
 
AsyncBin.prototype.isWaiting = function() {
 	for (var key in this._async) {
 		if (this._async.hasOwnProperty(key) && this._async[key]) {
 			return true;
 		}
 	}
 	
 	return false;
};
 
AsyncBin.prototype.reset = function() {
 	this._done = false;
 	
 	//mark all of the callback methods as not been completed
 	for (var key in this._async) {
 		if (this._async.hasOwnProperty(key)) {
 			this._async[key] = true;
 		}
 	}
};
 
 AsyncBin.prototype.ready = function(callback) {
 	if (callback!==undefined) {
 		this._callback = callback;
 	}
 	
 	//run the test once if it has not already been called
 	this._done = true;
 	
 	if (!this.isWaiting()) {
 		this._callback();
 	}
};
 
AsyncBin.prototype.callback = function(callbackId) {
 	if (callbackId===undefined) {
 		callbackId = this.add();
 	}
 	
 	var self = this;
 	
 	return function() {
 		if (self._async.hasOwnProperty(callbackId) && self._async[callbackId]) {
 			self._async[callbackId] = false;
 			
 			if (self._done && !self.isWaiting()) {
 				self._callback();
 			}
 		}
 	};
};
 
AsyncBin.prototype.remove = function(callbackId) {
 	delete this._async[callbackId];
};
 
AsyncBin.prototype.add = function() {
 	var callbackId = String(this._id++);
 	
 	this._async[callbackId] = true;
 	
 	return callbackId;
};

Usage

You can use the class below as follows:

var startTime = new Date().getTime();
var bin = new AsyncBin(done);
longTask(bin.callback());
longTask2(bin.callback());
bin.ready();
	
function done() { alert("Took " + (new Date().getTime() - startTime) + " ms"); };
function longTask(callback) { setTimeout(callback, 1000); };
function longTask2(callback) { setTimeout(callback, 2000); };

You can also create callback ids in advance and manually remove them from the bin if you wish:

var bin = new AsyncBin(function() {
	alert("done");
});

var done1 = bin.add();

longTask(bin.callback(done1));

bin.ready();

setTimeout(function() {
	done1.cancel(); //remove the callback method from the bin
	bin.ready(); //force the bin to execute its done method if the other items are also ready
}, 1000);