When using AJAX, an easy way to evaluate JavaScript or JSON returned from a web server is the good ol' eval function. However, if an error is encountered during the evaluation Microsoft's Script Debugger doesn't let you see the offending line. (I haven't checked what FireBug does yet.)
To get around this problem, you can insert the JavaScript as a <script> DOM node.
In other words, instead of this:
try this:
var scriptTag = document.createElement("script");
scriptTag.setAttribute('type', 'text/javascript');
scriptTag.text = js;
document.body.appendChild(scriptTag);
With the latter approach, the debugger works great.