GWT now includes AJAX Debugger
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Bruce Johnson and Dan Peterson of the Google Web Toolkit Team write:
In addition to making debugging far easier, GWT's unique compilation-based approach to AJAX has the nice property that it rewards developers for good software engineering practices. Java source code that is clear and organized can be easily optimized by the GWT compiler, which is a nice antidote to the frequent hack-and-slash approach that's all too common in JavaScript development. As your application grows, the GWT compiler begins to pay off in even bigger ways.