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

Question

In JavaScript, the "this" operator can refer to different things under different scenarios.

Typically in a method within a JavaScript "object", it refers to the current object.

But when used as a callback, it becomes a reference to the calling object.

I have found that this causes problems in code, because if you use a method within a JavaScript "object" as a callback function you can't tell whether "this" refers to the current "object" or whether "this" refers to the calling object.

Can someone clarify usage and best practices regarding how to get around this problem?

   function TestObject() {
            TestObject.prototype.firstMethod = function(){
                      this.callback();
                      YAHOO.util.Connect.asyncRequest(method, uri, callBack);

            }

            TestObject.prototype.callBack = function(o){
              // do something with "this"
              //when method is called directly, "this" resolves to the current object
              //when invoked by the asyncRequest callback, "this" is not the current object
              //what design patterns can make this consistent?
              this.secondMethod();
            }
            TestObject.prototype.secondMethod = function() {
             alert('test');
            }
        }

Answer 1

In JavaScript, this always refers to the object invoking the function that is being executed. So if the function is being used as an event handler, this will refer to the node that fired the event. But if you have an object and call a function on it like:

myObject.myFunction();

Then this inside myFunction will refer to myObject. Does it make sense?

To get around it you need to use closures. You can change your code as follows:

function TestObject() {
    TestObject.prototype.firstMethod = function(){
        this.callback();
        YAHOO.util.Connect.asyncRequest(method, uri, callBack);
    }            

    var that = this;
    TestObject.prototype.callBack = function(o){
        that.secondMethod();
    }

    TestObject.prototype.secondMethod = function() {
         alert('test');
    }
}

Answer 2

Quick advice on best practices before I babble on about the magic this variable. If you want Object-oriented programming (OOP) in Javascript that closely mirrors more traditional/classical inheritance patterns, pick a framework, learn its quirks, and don't try to get clever. If you want to get clever, learn javascript as a functional language, and avoid thinking about things like classes.

Which brings up one of the most important things to keep in mind about Javascript, and to repeat to yourself when it doesn't make sense. Javascript does not have classes. If something looks like a class, it's a clever trick. Javascript has objects (no derisive quotes needed) and functions. (that's not 100% accurate, functions are just objects, but it can sometimes be helpful to think of them as separate things)

The this variable is attached to functions. Whenever you invoke a function, this is given a certain value, depending on how you invoke the function. This is often called the invocation pattern.

There are four ways to invoke functions in javascript. You can invoke the function as a method, as a function, as a constructor, and with apply.

As a Method

A method is a function that's attached to an object

var foo = {};
foo.someMethod = function(){
    alert(this);
}

When invoked as a method, this will be bound to the object the function/method is a part of. In this example, this will be bound to foo.

As A Function

If you have a stand alone function, the this variable will be bound to the "global" object, almost always the window object in the context of a browser.

 var foo = function(){
    alert(this);
 }
 foo();

This may be what's tripping you up, but don't feel bad. Many people consider this a bad design decision. Since a callback is invoked as a function and not as a method, that's why you're seeing what appears to be inconsistent behaviour.

Many people get around the problem by doing something like, um, this

var foo = {};
foo.someMethod = function (){
    var that=this;
    function bar(){
        alert(that);
    }
}

You define a variable that which points to this. Closure (a topic all it's own) keeps that around, so if you call bar as a callback, it still has a reference.

As a Constructor

You can also invoke a function as a constructor. Based on the naming convention you're using (TestObject) this also may be what you're doing and is what's tripping you up.

You invoke a function as a Constructor with the new keyword.

function Foo(){
    this.confusing = 'hell yeah';
}
var myObject = new Foo();

When invoked as a constructor, a new Object will be created, and this will be bound to that object. Again, if you have inner functions and they're used as callbacks, you'll be invoking them as functions, and this will be bound to the global object. Use that var that = this; trick/pattern.

Some people think the constructor/new keyword was a bone thrown to Java/traditional OOP programmers as a way to create something similar to classes.

With the Apply Method.

Finally, every function has a method (yes, functions are objects in Javascript) named apply. Apply lets you determine what the value of this will be, and also lets you pass in an array of arguments. Here's a useless example.

function foo(a,b){
    alert(a);
    alert(b);
    alert(this);
}
var args = ['ah','be'];
foo.apply('omg',args);

Answer 3

this corresponds to the context for the function call. For functions not called as part of an object (no . operator), this is the global context (window in web pages). For functions called as object methods (via the . operator), it's the object.

But, you can make it whatever you want. All functions have .call() and .apply() methods that can be used to invoke them with a custom context. So if i set up an object Chile like so:

var Chile = { name: 'booga', stuff: function() { console.log(this.name); } };

...and invoke Chile.stuff(), it'll produce the obvious result:

booga

But if i want, i can take and really screw with it:

Chile.stuff.apply({ name: 'supercalifragilistic' });

This is actually quite useful...

Answer 4

If you're using a javascript framework, there may be a handy method for dealing with this. In Prototype, for example, you can call a method and scope it to a particular "this" object:

var myObject = new TestObject();
myObject.firstMethod.bind(myObject);

Note: bind() returns a function, so you can also use it to pre-scope callbacks inside your class:

callBack.bind(this);

http://www.prototypejs.org/api/function/bind

Answer 5

I believe this may be due to how the idea of [closures](http://en.wikipedia.org/wiki/Closure_(computer_science) work in Javascript.

I am just getting to grips with closures myself. Have a read of the linked wikipedia article.

Here's another article with more information.

Anyone out there able to confirm this?

Answer 6

You can also use Function.Apply(thisArg, argsArray)... Where thisArg determines the value of this inside your function...the second parameter is an optional arguments array that you can also pass to your function.

If you don't plan on using the second argument, don't pass anything to it. Internet Explorer will throw a TypeError at you if you pass null (or anything that is not an array) to function.apply()'s second argument...

With the example code you gave it would look something like:

YAHOO.util.Connect.asyncRequest(method, uri, callBack.Apply(this));

Answer 7

If you're using Prototype you can use bind() and bindAsEventListener() to get around that problem.

Answer 8

As soon as callback methods are called from other context I'm usually using something that I'm call callback context:

var ctx = function CallbackContext()
{
_callbackSender
...
}

function DoCallback(_sender, delegate, callbackFunc)
{
 ctx = _callbackSender = _sender;
 delegate();
}

function TestObject()
{
   test = function()
  {
     DoCallback(otherFunc, callbackHandler);
  }

  callbackHandler = function()
{
 ctx._callbackSender;
 //or this = ctx._callbacjHandler;
}
}