Look here when Chapter 3 of the book mentions something extra to be found on the web site.
The sites below have all sorts of free, pre-written scripts to do fancy things in Web pages. By reading the documentation, it is usually not too hard to get them working in your own pages.
subsection in Section 3.2
of the text for how to use those. However,
Mozilla's Venkman Debugger
is a full-featured debugger similar to what you are used to in a JAVA IDE, for example. Devedge has good article
about the debugger.