In Javascript, the key work "this" is a hard point to understand, in this artical, I will try to explain it

talking about "this", we have to talk about closure and the way function is called in Javascript.

function have for kinds of calling:

1. called as a method:

1     var s = {2         a : ‘123‘,3         foo:function(){4             return this.a5         }6     }7     8     s.foo() //a

in this code, the function is called as object‘s method, in the closure of the function ,this point to its parent object.

2. called as a function

1     var foo = function(){2         return this.a3     }4     foo() // if a is define in the gloable, return a, else, return undefined.

 in this code, the function is called as a function, in the code‘s closure, this will point to the gloable window.

3. called as a constructor

1     var Foo = function(){2         this.a = 1;3         this.b = 2;4         return this;5     }6     7     var foo = new Foo();8     foo.a //1;

 in this code, the function is called as a constructor, this will point to the new object.

4. called by .apply and .call

1     var foo = function(arg1){2         this.a = arg1;3     }4     var s = {};5     foo.apply(s,[1]);

 in this code, when the apply is called, this will be binded to the first param of the apply.