JavaScript – JavaScript in Web Browsers

The first part of this book described the core JavaScript
language. We now move on to JavaScript as used within web browsers,
commonly called client-side JavaScript. Most of the examples we’ve seen
so far, while legal JavaScript code, have no particular context; they
are JavaScript fragments that run in no specified environment. This
chapter provides that context.

Before we begin talking about JavaScript, it is worth thinking
about the web pages we display in web browsers. Some pages present
static information and can be called documents. (The presentation of
that static information may be fairly dynamic—because of JavaScript—but the information
itself is static.) Other web pages feel more like applications than
documents. These pages might dynamically load new information as needed,
they might be graphical rather than textual, and they might operate
offline and save data locally so they can restore your state when you
visit them again. Still other web pages sit somewhere in the middle of
the spectrum and combine features of both documents and

This chapter begins with an overview of client-side JavaScript. It
includes a simple example and a discussion of the role of JavaScript in
both web documents and web applications. That first introductory section
also explains what is coming in the Part II chapters
that follow. The sections that follow explain some important details
about how JavaScript code is embedded and executed within HTML
documents, and then they introduce the topics of compatibility,
accessibility, and security.

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