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I would love something like seymour for JS development. Being able to see the complexity of execution as you type would be invaluable.

Exactly this. I'm a very visual person in regards to getting things done. Being able to see how a function works, would be delightful.

Not a perfect solution, but this allows you to see how the computer executes a JS code. Hit the forwards and back buttons: http://pythontutor.com/live.html#code=function%20nested%28%2...