The window object

　　At the core of the BOM is the window object, which represents an instance of the browser. The window object serves a dual purpose in browsers, acting as the JavaScript interface to the browser window and the ECMAScript Global object. This means that every object, variable, and function defined in a web page uses windows as its Global object and has access to methods like parseInt().

The Global Scope

　　Since the window object doubles as the ECMAScript Global object, all variables and functions declared globally become properties and methods of the window object. Consider this example:

 var age = 29;
 function sayAge(){
   alert(this.age);
 }

 alert(window.age);
 sayAge();
 window.sayAge();

　　Despite global variables becoming properties of the window object, there is a slight difference between defining a global variable and defining a property directly on window: global variables cannot be removed using the delete operator, while properties defined on window can. For example:

 var age = 29;
 window.color = "red";

 // throws an error in IE < 9, returns false in all other browsers
 delete window.age;

 // throws an error in IE < 9, returns true in all other browsers
 delete window.color;

 alert(window.age);          //
 alert(window.color);        // undefined

　　Properties of window that were added via var statements have their [[Configurable]] attribute set to false and so may not be removed via the delete operator.

　　Another thing to keep in mind: attempting to access an undeclared variable throws an error, but it is possible to check for the existence of a potentially undeclared variable by looking on the window object. For example:

 // this throws an error because oldValue is undeclared
 var newValue = oldValue;

 // this doesn't throw an error, because it's a property lookup
 // newValue is set to undefined
 var newValue = window.oldValue;

　　Keeping this in mind, there are many objects in JavaScript that are considered to be global, such as location and navigator(both discussed later in the chapter), but are actually properties of the window object.