JavaScript ---- The Global Object
1. The global object is a regular JavaScript object that serves a very important purpose: the properties of this object are the globally defined symbols that are available to a JavaScript program.
When the JavaScript interpreter starts(or whenever a web browser loads a new page), it creates a new global object and gives it an initial set of properties that define:
global properties like undefined, Infinity, and NaN
global functions like isNaN(), parseInt() and eval()
constructor functions like Date(), RegExp(), String(), Object(), and Array()
global objects like Math and JSON


2. In top-level code----JavaScript code that is not part of a function----you can use the JavaScript keyword this to refer to the global object:

var global = this;     // Define a global variable to refer to the global object

3. In client-side JavaScript, the Window object serves as the global object for all JavaScript code contained in the browser window in represents. This global Window object has a self-referential window property that can be used instead of this to refer to the global object.