Professional, JavaScript® for Web Developers, Second Edition

Chapter 8. The Browser Object Model

Though ECMAScript describes it as the core of JavaScript, the Browser Object Model (BOM) is really the core of using JavaScript on the Web. The BOM provides objects that expose browser functionality independent of any web page content. A lack of any real specification makes the BOM both interesting and problematic, because browser vendors are free to augment it as they see fit. The commonalities between browsers are de facto standards that have survived browser development mostly for the purpose of interoperability. There is no such thing as a standard BOM implementation or standard BOM interfaces.

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 as well as the ECMAScript Global object. This means that every object, variable, and function defined in a web page uses window 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);    //29
sayAge();             //29
window.sayAge();      //29

Here, a variable named age and a function named sayAge() are defined in the global scope, which automatically places them on the window object. ...