simple queueing of async-functions

async-queue.js

var _queue = [];
function enqueue(data, itemProcessor, callback) {
    for(var i=0, n=data.length; i<n; i++) {
        (function(item) {
            _queue.push(function(next) {
                itemProcessor(item, function() {
                    window.setTimeout(next, 0);
                });
            });
        } (data[i]));
    }

    return function next() {
        (_queue.shift() || callback || function() {}) (next);
    };
}


// sample-usage:
function async1(item, callback) {
    console.log('async1: start processing item ' + item);
    window.setTimeout(function() {
        console.log('async1: completed processing item ' + item);
        callback();
    }, 500);
}

function async2(item, callback) {
    console.log('async2: start processing item ' + item);
    window.setTimeout(function() {
        console.log('async2: completed processing item ' + item);
        callback();
    }, 500);
}


var tbd = ['a', 'b', 'c', 'd', 'e', 'f'];

var runQueue = enqueue([1,2,3,4], async1, function(next) {
    console.log("queue-run finished.");
    if(0 == tbd.length) {
        console.log("all done. quit.");
        return;
    }

    var remaining = tbd.splice(0,3);
    enqueue(remaining, async2);

    next();
});

runQueue();