this operator in javascript

Question

Suppose I have JavaScript code like

      myClass = function(){
          function doSomething(){
              alert(this); // this1 
          }
      } 
      alert(this); //this2

What those two 'this' objects are refer for??

Answer 1

The this value in the global execution context, refers to the global object, e.g.:

this === window; // true

For Function Code, it really depends on how do you invoke the function, for example, the this value is implicitly set when:

Calling a function with no base object reference:

myFunc();

The this value will also refer to the global object.

Calling a function bound as a property of an object:

obj.method();

The this value will refer to obj.

Using the new operator:

new MyFunc();

The this value will refer to a newly created object that inherits from MyFunc.prototype.

Also, you can set explicitly that value when you invoke a function, using either the call or apply methods, for example:

function test(arg) {
  alert(this + arg);
}
test.call("Hello", " world!"); // will alert "Hello World!"

The difference between call and apply is that with apply, you can pass correctly any number of arguments, using an Array or an arguments object, e.g.:

function sum() {
  var result = 0;
  for (var i = 0; i < arguments.length; i++) {
    result += arguments[i];
  }
  return result;
}

var args = [1,2,3,4];
sum.apply(null, args); // 10

// equivalent to call
sum(1,2,3,4); // 10

If the first argument value of call or apply is null or undefined, the this value will refer to the global object.

_{(note that this will change in the future, with ECMAScript 5, where call and apply pass the thisArg value without modification)}