Zerø Wind Jamie Wong

AJAX Method Callbacks and Omegle Voyeur Update

I finally got back around to updating Omegle Voyeur with the ability to interfere, and decided to re-implement the whole thing in jQuery while I’m at it. Since jQuery doesn’t come with a built-in method of building classes, I used lowpro for jQuery. It’s a port of a class building scheme from Prototype. It doesn’t do everything I could have hoped for, but it served most of my needs.

The other thing I implemented was a way of knowing when Omegle is blocking requests. They have a more robust form of detection now - it isn’t just manual IP ban. Once you request too many things from them too fast, they start requesting a captcha. Locally, this isn’t a problem - I simply embed an iframe with Omegle in it and provide instructions to the user. Hosted, this is a more troublesome problem, since the captcha is directed towards an IP, so it must be responded to from that IP. I have no solution to this problem at the moment, but I’m going to look into implementing the whole thing using Greasemonkey so this isn’t an issue at all.

For now, you can see the latest version here: Omegle Voyeur. Don’t be surprised if it’s down, and please go grab your own copy: Omegle-Voyeur @ github.

Now on to the customary technical concept to go along with my own self promotion.

AJAX Method Callbacks

While passing functions as arguments is a pretty standard thing among almost all languages, attempting to pass methods of specific instances as arguments in JavaScript presents an interesting problem. Consider the following:

function car(price) {
    this.price = price;

    this.setPrice = function(price) {
        this.price = price;
    };
}

function pass666(func) {
    func(666);
}

var redcar = new car(2000);
alert(redcar.price);
redcar.setPrice(123);
alert(redcar.price);
pass666(redcar.setPrice);
alert(redcar.price);

As you might expect, the first two alerts will say 2000 and 123 respectively. But the last one also says 123. Why?

It all has to do with what this refers to. Both in the initialization of redcar and the modifier call redcar.setPrice, “this” refers to the instance of the function car given the identifier name redcar. In the pass666 version, this refers to the function pass666. As a result, it does nothing to modify the properties of the car because it isn’t told anything about redcar.

One way to fix this is to use a placeholder variable. I used “self”. Change the definition of car to the following yields the desired result.

function car(price) {
    this.price = price;

    var self = this;
    this.setPrice = function(price) {
        self.price = price;
    };
}

In this example, it’s difficult to see why you would ever want to use this in the first place. The reason I encountered this problem is my need to use instance methods as callback functions for AJAX calls. Here’s an excerpt of the jQuery version of Omegle Voyeur to see what I’m talking about.

sendQuery: function(target,respFunc) {
  // Send a query to the omegle server
  var self = this;
  if (respFunc == null) {
    respFunc = function(self,data) {}
  }
  $.ajax({
    url: 'omegle.php?'+target,
    type: 'GET',
    dataType: 'json',
    success: function(data) {
      respFunc(self,data);
    }
  });
},

Sending a request to the Omegle server is a very common task in Omegle Voyeur, so I wanted all the AJAX requests leaving from the same method. This means I have to accept the callback function as a parameter. I’ve written all the callback methods to accept a parameter self which will refer to the instance of interest.

This aspect is one of the many things that makes Prototype’s class system superior to jQuery’s. However, since jQuery makes a lot of other things nicer and the two libraries don’t play together very well, I decided to port over to jQuery nonetheless. In Prototype, there’s a function called bind (not to be confused with jQuery’s bind which does something completely different,) which solves this problem elegantly.

To fix the redcar problem with the aid of Prototype without having to use a placeholder variable, you can use bind like so:

function car(price) {
    this.price = price;

    this.setPrice = function(price) {
        this.price = price;
    };
}

function pass666(func) {
    func(666);
}

var redcar = new car(2000);
alert(redcar.price);
redcar.setPrice(123);
alert(redcar.price);

pass666(redcar.setPrice.bind(redcar));

alert(redcar.price);

The line pass666(redcar.setPrice.bind(redcar)); is what makes this work out. We’re explicitly saying that we want setPrice executed from the scope of the instance.

If I ever have to hire someone for a web development job, I’ll be sure to ask something about this.

If you liked reading this, you should follow me on twitter, take a look at other blog posts by me, or maybe even come work with me at Figma!


Zerø Wind Jamie Wong
Previously Learn Source Control with Git March 9, 2010