Question

Possible Duplicate:
In Javascript, why is the “this” operator inconsistent?

I have the following class:

function Chat(some, nick, url) {
    this.socket = null;
    this.Nickname = nick;
    this.Url = url;

    this.Connect = function () {
        socket = io.connect(this.Url);
        socket.on('connect', function (data) {
            var p = this.Nickname; //this.Nickname is undefined why? 
            // how can I acess to the Nickname variable or function?
        }
    };
}

How can I acces the instance variable or function from the connect callback function?

Answer 1

The simplest solution is to use the that trick

var that = this; //that is a normal variable
                 //so it is lexically scoped
                 //and can be used in inner functions

socket.on('connect', function(data){
    var p = that.NickName;
});

Another possibility is explicitily binding the correct this to the callback function

socket.on('connect', function(data){
    var p = this.Nickname;
}.bind(this));

The that trick has the advantage of nesting to as many callbacks as you want and the bind version has the advantage of allowing you to still use “this” inside.

A disadvantage of the bind method is that it is not supported in IE<=8 so you might need to use a shim if you need to support ancient browsers.

edit: This answer is a bit old. Nowadays you probably don’t need to worry about IE6 anymore and you might be able to use fat arrow syntax, which doesn’t overwrite the this.