Handling of multiple parallel async resolutions in one main callback.

parallela.js

/**
 * Handling of multiple parallel async resolutions in one main callback.
 *
 * @param  {Function} callback Function executed after all async stuff has finished.
 *
 * @return {Function} Proxy generator.
 */
function parallela(callback) {
	var args = {};
	var proxies = 0;

	/**
	 * Generate and register a proxy that has to be executed
	 * for final callback to be fired.
	 *
	 * @param  {String}  name  Name of the object key for storing arguments.
	 * @param  {Integer} argNo Index of argument to save. Omit to save all.
	 *
	 * @return {Function} Proxy.
	 */
	return function (name, argNo) {
		if (typeof name !== 'string') {
			name = false;
		}
		proxies++;

		/**
		 * Proxy function that saves needed argument,
		 * and executes final callback when last in queue.
		 *
		 * @return {Void}
		 */
		return function () {
			if (name) {
				args[name] = argNo == null ? arguments : arguments[argNo];
			}
			if(!--proxies) {
				callback(args);
			}
		};
	};
}

usage.js

/**
 * Simulating async cb execution.
 *
 * @param  {Mixed}    ret Value to be returned asynchronously.
 * @param  {Function} cb  Callback that will receive the value.
 *
 * @return {Void}
 */
function hey(ret, cb) {
	setTimeout(function () {
		cb(null, ret);
	}, Math.random()*90+10);
}

// Create a new paralella and save the proxy generator.
var proxy = parallela(function (result) {
	console.log(result);
});

// Simulate async execution of callbacks from proxy generator.
hey('this is foo', proxy('foo', 1));
hey('this is bar', proxy('bar'));    // note: unspecified argNo
hey('this is baz', proxy('baz', 1));

// If you don't want to save any arguments, just create an empty proxy.
hey('none will see this', proxy());

/*
When all hey()s have finished, console.log from main callback will report this arguments map:
{
	foo: 'this is foo',
	bar: [
		null,
		'this is bar'
	],
	baz: 'this is baz'
}

You can omit `argNo` argument in proxy generator and save the whole
`arguments` array if you need multiple arguments, or error handling.
*/