this keyword problem in JavaScript

<script type="text/javascript" src="https://getfirebug.com/firebug-lite.js"></script>
<script>
(function(){
    var myLib = function (selector) {
        console.log(this);
        if(this === window) {
            console.log('first if statement executed');
            console.log(this);
       开发者_开发百科     return new myLib(selector);
        }
        if(this.window === window){
            console.log('second if statement executed');
            console.log(this);
            return new myLib(selector);
        }
    };

    //expose myLib to the global window object
    window.myLib = myLib;
})();

myLib('.someclass')

</script>

In Firefox, this is what gets logged to the console:

Window

first if statement executed

Object {}

In IE8, this is what gets logged to the console:

Window

second if statement executed

Window

Object {}

Why does this not point to the window in IE when myLib is invoked, but instead this.window points to the window?

window is weird. It's both the window object and the global scope object.

Since window is the [[Global Context]] then this works:

var o = 5;
window.o === 5; // true

Since window is a global variable then window.window === window

The fact that in IE8 window !== this is because IE8 hates you. IE8 really messed up the way it implemented window as global context and as a global variable.

To solve your code problem change

window.myLib = myLib;

to

window.myLib = function() {
    return new myLib();
};

if you're trying to avoid calling myLib without invoking a constructor function you can do something like this:

function myLib(){
  if(!(this instanceof myLib)){
    return new myLib();
   }
}