JavaScript: The Definitive Guide, Fourth Edition

Execution of JavaScript Programs

The previous section discussed the mechanics of integrating JavaScript code into an HTML file. Now we move on to discuss exactly how that integrated JavaScript code is executed by the JavaScript interpreter. The following sections explain how different forms of JavaScript code are executed. While some of this material is fairly obvious, there are a number of important details that are not so obvious.

Scripts

JavaScript statements that appear between <script> and </script> tags are executed in order of appearance; when more than one script appears in a file, the scripts are executed in the order in which they appear. If a script calls document.write( ), any text passed to that method is inserted into the document immediately after the closing </script> tag and is parsed by the HTML parser when the script finishes running. The same rules apply to scripts included from separate files with the src attribute.

The detail that is not so obvious, but is nevertheless important to remember, is that execution of scripts occurs as part of the web browser’s HTML parsing process. Thus, if a script appears in the <head> section of an HTML document, none of the <body> section of the document has been defined yet. This means that the JavaScript objects that represent the contents of the document body, such as Form and Link, have not been created yet and cannot be manipulated by that code.

Your scripts should not attempt to manipulate objects that have not ...