Question

In some of my Javascript objects I find that my this pointer is correct – these are new Func() -type objects – when created, but in the assigned methods it can be wrong.

function Confused() {
   console.log("checking",this==window,"is always false");
   this.method = function() {
       console.log("checking",this==window,"is true for some funcs but not others");
   };
};

On some calls to (new Confused()).method() – it seems to have lost it’s this pointer. The times this happen seem dependent on the function, rather than random; its something in the code around how I’m creating classes that is causing this.

An example is online at http://williame.github.com/barebones.js/ and the member callback G3D._file_loaded has a wrong this pointer when called sometimes.

Why, and how do I fix it?

Answer 1

There are 4 ways to use a function in Javascript
what each of these does is change what the content of this is :

• function calls: this = global object (window in browser)
• method calls: this = object it is called from.
• constructor calls: this = new object you are creating.
• call/apply calls: this = object you passed.

In your case this == window when you call the function directly (Confused()) but if you call using new (new Confused()) then it will be the new object you are creating.