JavaScript: The Definitive Guide, Fourth Edition

Chapter 12. JavaScript in Web Browsers

The first part of this book described the core JavaScript language. Now we move on to JavaScript as used within web browsers, commonly called client-side JavaScript.^[43] Most of the examples we’ve seen so far, while legal JavaScript code, had no particular context; they were JavaScript fragments that ran in no specified environment. This chapter provides that context. It begins with a conceptual introduction to the web browser programming environment and basic client-side JavaScript concepts. Next, it discusses how we actually embed JavaScript code within HTML documents so it can run in a web browser. Finally, the chapter goes into detail about how JavaScript programs are executed in a web browser.

The Web Browser Environment

To understand client-side JavaScript, you must understand the conceptual framework of the programming environment provided by a web browser. The following sections introduce three important features of that programming environment:

• The Window object that serves as the global object and global execution context for client-side JavaScript code

• The client-side object hierarchy and the document object model that forms a part of it

• The event-driven programming model

The Window as Global Execution Context

The primary task of a web browser is to display HTML documents in a window. In client-side JavaScript, the Document object represents an HTML document, and the Window object represents the window (or frame) that displays ...