关于全局对象的几个问题

Question

A global object is an object that always exists in the global scope. In JavaScript there is always a global object defined.

Can there be more than one global object? Why do people say Window is a global object when MDN doesnt say this?

Answer 1

Every browser window has a single top level Window object routinely referred to as global object . The global object is accessible via window variable.
The object exposes a number of properties, most importantly document which is also an object, methods, and events.
A developer may define (via script) variables ( var ) and function s which automatically become properties and methods of window ( global ) object (provided they are declared on top (global) level).
For example, an expression var a = 1; declares a variable named a (and assign numeric value to it). This variable is accessible by its name ( a ) and as well as window.a and window['a'] .
The reference to the global window object can be omitted so window.a = 2; and a = 2; are equivalent (but not ['a'] = 2; which is syntax error).
Also note that global window object belongs to the browser window, not to javascript. The browser only expose it to Scripting Engine .

This is javascript basics.

I hope this explanation clears a little your confusion.