Async Limited Queue

NQ.js

var NQ = { 
n: 1
,q: []

,add: function add(fn) { this.push(fn); this.next(); return this; }
,push: function push(fn) { this.q.push(fn); return this; }
,fire: function fire() { if (!this.q.length) return "empty"; this.run(this.q[0]); 
this.q.shift(); return this.q.length; }
,cancel: function cancel() { this.q.length = 0; }

,success: function success() { console.log(arguments); }
,fail: function fail() { console.warn.apply(console, arguments); }

,cb: function cb() { NQ.success(arguments); NQ.fire(); }
,err: function err() { NQ.fail(arguments); NQ.fire(); }
,next: function next() { if (this.q.length < this.n) return this.fire(); else return "blocked"; }
,run: function run(fn) { return fn(NQ.err,NQ.cb); }
};

/* Based on suggestions from http://stereopsis.com/async/ */

/* Example usage: */
function goody(oops, ok) { setTimeout(function() {return ok(200 + "goody");}, 500); }

function baddy(oops, ok) { var start = new Date(); return setTimeout(function() { oops(NQ.q.length, "baddy", new Date().toString() ); },  1000) }

NQ.push(goody); NQ.add(goody).next(); NQ.add(baddy).add(baddy).add(baddy).add(baddy).add(baddy).add(goody).add(baddy).push(goody).fire();