JavaScript: The Definitive Guide, Fourth Edition

Variables as Properties

You may have noticed by now that there are a lot of similarities in JavaScript between variables and the properties of objects. They are both assigned the same way, they are used the same way in JavaScript expressions, and so on. Is there really any fundamental difference between the variable i and the property i of an object o? The answer is no. Variables in JavaScript are fundamentally the same as object properties.

The Global Object

When the JavaScript interpreter starts up, one of the first things it does, before executing any JavaScript code, is create a global object. The properties of this object are the global variables of JavaScript programs. When you declare a global JavaScript variable, what you are actually doing is defining a property of the global object.

The JavaScript interpreter initializes the global object with a number of properties that refer to predefined values and functions. For example, the Infinity, parseInt, and Math properties refer to the number infinity, the predefined parseInt( ) function, and the predefined Math object. You can read about these global values in the core reference section of this book.

In top-level code (i.e., JavaScript code that is not part of a function), you can use the JavaScript keyword this to refer to the global object. Within functions, this has a different use, which is described in Chapter 7.

In client-side JavaScript, the Window object serves as the global object for all JavaScript code ...