This should really be Tip #1 since it’s the most critical of all. Let me just say this as clearly as possible:

Your fastest Javascript projects are the ones that have the least Javascript!

Sure, Javascript engines have matured. Safari’s Nitro, Chrome’s V8, Firefox’s TraceMonkey, and Opera’s Carackan all kick some serious butt. (IE8’s JScript, unfortunately, still sucks wind.) However, routines written in C++ still run orders of magnitude faster in most cases.

Don’t write Javascript to do something your browser already does at compiled-code speeds.
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