With most web applications and dynamic websites, the program logic is on the server. A program written in PHP, Java, Python, or Perl generates HTML pages tailored to user requests. However, the browser itself includes a complete and very powerful run-time environment with its JavaScript interpreter.

JavaScript programs respond to user input and access the opened HTML page via a so-called DOM API. This interface between JavaScript and HTML lets you change attributes for each tag on the page, such as color, size, or visibility. It also lets you remove sections of a page or insert HTML elements anywhere (Figure 1).

Old Friends

Apart from being object oriented, JavaScript is very similar to the C programming language and shares many of the basic structures (loops, conditions, operators) of other languages, from Java to Perl. That's what makes the switchover so easy, as long as you use only the given procedural elements of the language that arrange the source code into routines to satisfy specific tasks.

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