simple semaphore for parallel async execution, with error handling.

semaphore.js

function queue(name){
	queue.q[name]++ || (queue.q[name] = 1);
	return function(err){
		if (err && queue.e[name]) queue.e[name](err);
		else if (err) throw err;
		process.nextTick(function(){
			queue.q[name]--;
			queue.check(name);
		});
	}
}
queue.__proto__ = {
	q: {},
	c: {},
	e: {},
	check: function(name){ queue.q[name] == 0 && queue.c[name](); },
	done: function(name, fn){ queue.c[name] = fn; },
	err: function(name, fn){ queue.e[name] = fn; }
}

/* -- test -- */

// mock async function
function somethingAsync(s, cb){
	setTimeout(function(cb){ console.log(s); cb(); }, Math.random()*1000, cb);
};

somethingAsync('frist', queue('bacon'));
somethingAsync('second', queue('bacon'));
somethingAsync('third', queue('bacon'));

// throw an error along the way
setTimeout(function(){
	queue('bacon')(new Error('damn it leroy'));
}, 200);

// called when all queued functions are done
queue.done('bacon', function(){
	console.log('done!');
});

// error handler
queue.err('bacon', function(err){
	console.log('something went wrong :(', err);
	//throw err;
});

console.log('let\'s see:');