A document [1] by Ray Cromwell introduces an aid for JS compression, both theory and implementation [2]. The idea is to transform JS code before compression in a way that, while keeping JS semantics, makes is best possible candidate for the popular gzip deflate compression.
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[1] http://timepedia.blogspot.com/2009/08/on-reducing-size-of-co...
[2] http://code.google.com/p/google-web-toolkit/source/detail?r=...