Node.js semaphores

semaphore.js

var Semaphore = function(callback, context) {
    this.semaphore = 0;
    this.callback = callback;
    this.context = context || this;
};

Semaphore.prototype.increment = function() {
    this.semaphore++;
};

Semaphore.prototype.execute = function() {
    this.semaphore--;
    if (this.semaphore <= 0 && this.callback) {
        this.callback.apply(this.context, arguments); //this means that the args that actually reach the callback will be from the LAST async block to call .execute();
    }
};

module.exports = Semaphore;

test_semaphore.js

var Semaphore = require("semaphore"),
    sys = require("sys");

var sem1 = new Semaphore(function(i) {
    sys.puts("ultimate callback... iteration that executed last = " + i);
});

//create a bunch of async ops, and we only want the sys.puts() call to happen after all have finished
for (var i = 0, l = 100; i < l; i++) {
    //before each async op, explicitly tell sem1 to increment internal counter
    sem1.increment();
    var timeout = Math.floor(Math.random()*1001); //random amount of time, up to 1 second
    setTimeout((function(_i) { //close over i so we can see which iteration ends up being the last to timeout/execute
        return function () { sem1.execute(_i); };
    })(i), timeout);
}