Question

I want to know what the global object in JavaScript is and to which class this object belongs to.

And how are Infinity, NaN and undefined part of the global object?

Answer 1

Variable scope is defined in JavaScript by a function, and functions can be nested inside other functions.

function foo() {
    // new variable scope in here
    var a = "a";

    function bar() {
       // another nested variable scope
       var b = "b";
    }
    bar();
}
foo();

EXCEPT there is a default “global” variable scope that is defined when your program runs. It is the base variable scope in which all function created scopes are nested.

So what?

Well, every variable scope has a variable object (or more accurately, a “binding” object). It’s an internal object to which all the local variables you create are bound.

This variable object is not directly accessible. You can only add properties to it by declaring a local variable (or function parameter, or function declaration). And you can only access properties via the variable names.

Again, so what?

Well the “global” variable scope is unique. It exposes this internal variable object by automatically defining a property on the object that refers back to the object itself. In a browser, the property is named window.

Because a property is placed on the object that refers back to the object, and because properties on the object become variables, we now have a direct access to the global variable object.

You can test this by observing that the window.window property is an equal reference to the window variable.

alert(window.window === window); // true

As a result, we can add a property to the object window.foo = "bar";, and it show up as a global variable alert(foo); // "bar".

Note that the only variable scope that exposes this internal object is the global scope. None of the function scopes expose it.

Also note that the ECMAScript specification does not require that the global variable object be exposed. It is up to the implementation to decide.