function somethingAsync(item, fn) {
	setTimeout(function() {
		fn(item * 2);
	}, Math.random() * 1000);
}

function processQueue(parallel, queue, cb) {
	var index = 0;
	var pending = 0;
	var processed = [];

	function dequeue() {
		var item = queue.shift();
		if( item ) {
			var position = index;
			index++;
			pending++;
			somethingAsync(item, function(result) {
				processed[position] = result;
				pending--;

				if( queue.length ) {
					dequeue();
				} else if( pending === 0) {
					cb( processed );
				}
			});
		}
	}

	for(var i = 0; i < parallel && i < queue.length; i++) {
		dequeue();
	}
}

processQueue(5, [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20], function(processed) {
	console.log(processed);
});

// [2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36, 38, 40]