If you’re looking for a development environment for JavaScript, JavaScript debuggers are available as a free download from both Microsoft and Netscape. The feature set of the two products is similar, although Microsoft’s debugger will also allow you to step through scripting done in other languages, such as VBScript. I recommend downloading both programs as you’re likely to encounter problems that crop up on one platform or the other, but not necessarily both.
Microsoft Script Debugger 1.0:
http://www.microsoft.com/scripting/debugger/default.htm
Netscape JavaScript Debugger 1.1:
http://developer.netscape.com/software/tools/index.html?content=/software/jsdebug.html