【The Global Object】
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() (§3.8.2), and eval() (§4.12).
• constructor functions like Date(), RegExp(), String(), Object(), and Array()
(§3.8.2)
• global objects like Math and JSON (§6.9)
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 In client-side JavaScript, the Window object serves as the global object for all JavaScript code contained in the browser window it represents. This global Window object has a self-referential window property that can be used instead of this to refer to the global object. The Window object defines the core global properties, but it also defines quite a few other globals that are specific to web browsers and client-side JavaScript.
