JavaScript® Bible, Sixth Edition

Chapter 45. Debugging Scripts

IN THIS CHAPTER

• Identifying the type of error plaguing a script

• Interpreting error messages

• Preventing problems before they occur

One of the first questions that an experienced programmer asks about a programming environment is what support is there for debugging code. Even the best coders in the world make mistakes when they draft programs. Sometimes, the mistakes are a mere slip of a finger on the keyboard; other times, they result from not being careful with expression evaluation or object references. The cause of the mistake is not the issue: finding the mistake and getting help to fix it is.

Some debugging tools are available for the latest browsers. For the most part, they have come from the browser makers themselves, or they are tied very closely to a particular authoring environment. Some of these tools are very quirky; others require significant investments in authoring environments. Discussion about debugging tools in this chapter, however, focuses on simple tools you can download online. By understanding the true meaning of error messages and working out the problem with the tools provided here, you should be able to overcome your bugs.

Syntax versus Runtime Errors

As a page loads into a JavaScript-enabled browser, the browser attempts to create an object model out of the HTML and JavaScript code in the document. Some types of errors crop up at this point. These are mostly syntax errors, such as failing to include a closing brace after a function's ...