parallel async JS micro library in 25LOC

allDone.js

var allDone = function(done){ // takes a callback that will be called as callback(errors, values) when all async parallel operations are finished.
	var context = {task: {}, data: {}};
	context.end = function(e,v){ return done(e,v), done = function(){} }; // this can always be called if you want to terminate early, like because an error.
	context.add = function(fn, id){ // if the async operation you are doing replies with standard fn(err, value) then just pass in a string ID, else your own callback and ID.
		context.task[id = (typeof fn == 'string')? fn : id] = false;
		var next = function(err, val){
			context.task[id] = true; // good, we're done with this one!
			if(err){ (context.err = context.err || {})[id] = err } // record errors.
			context.data[id] = val; // record the values.
			for(var i in c.task){ if(c.task.hasOwnProperty(i)){ // loop over the async task checker
				if(!c.task[i]){ // if any of them are still false
					i = true; 
					break; // then break and...
				} 
			} }
			if(!(i === true)){ // only call done if i was never assigned to true, indicating there was no task left.
				done(context.err, context.data);
			}
		}, c = context;
		return (fn instanceof Function)? function(){ // if we have a callback, proxy it.
			return fn.apply({task: c.task, data: c.data, end: c.end, done: next}, arguments);
		} : next; // else just handle it ourselves.
	}
	return context;
};

test.js

// use:
[2, 3, 4, 9].forEach(function(num, i){
	setTimeout(this.add(function(){ // async operation here, proxied via adding another task to allDone.
		console.log('async doing', num);
		this.done(null, num * num); // done done done... endlessly, until we are actually done.
	}, i), 20); // the ID of i is the index of the array, and 20 is just a random number we set for the setTimeout, to get async behavior.
}, allDone(function(err, val){
	console.log("all done!", err, val);
}));

If you like this, you'll probably like my newest open source project, GUN: https://github.com/amark/gun .