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Developer builds of Chrome now support a simple remote debugging protocol - JSON RPC over WebSockets. This protocol provides much of what is needed to build high quality debugging tools for ClojureScript. Coupled with source-mapping and analysis, it should be possible to develop sexpr level stepping and debugging of ClojureScript source.
Rhino Debugging API
http://www.mozilla.org/rhino/rhino15R4-debugger.html
V8 Debugging API
http://code.google.com/p/v8/wiki/DebuggerProtocol
References
http://www.webkit.org/blog/1620/webkit-remote-debugging/
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