JavaScript Performance For Madmen

JavaScript performance is terrifying and unpredictable. If you want things to run fast, you'll need a dowsing rod, but these test cases might help:

• Object.defineProperty versus regular property assignment
• Performance hit for initializing properties in different orders in your constructor (Hidden Classes)
• Deeply nested prototype chains
• String concatenation versus array.join
• Truncating numbers
• Object.create vs new

Performance minutiae for individual JS runtimes

SpiderMonkey/TraceMonkey/IonMonkey/JaegerMonkey (Mozilla Firefox)

• Functions containing try/finally are run by the interpreter.

V8 (Google Chrome)

• See http://www.youtube.com/watch?v=XAqIpGU8ZZk for various tips
• V8 cannot represent integers larger than 31 bits as an integer (they get promoted to floats).
• Floating-point values are almost universally stored in the heap by V8 (which means each one is an allocation).
• Any function containing a try/catch or try/finally is not optimized.
• Functions that are too long (including comments and newlines/whitespace) are not inlined and may not be optimized.

Vaguely useful profiling tools

Mozilla Firefox

• SPS (Currently only supports native profiling; JS profiling support forthcoming. Lets you share profiles on the web with other people! Amazing!)
• JIT Inspector (Completely inscrutable unless you read this PDF, at which point it is only partially inscrutable. Also, doesn't display actual numbers anywhere or let you save profiles...)
• Firebug (if you can get the profiler to work instead of crashing the browser entirely)

Google Chrome

• Web Inspector's Profiler tab (This is a sampling profiler with poor accuracy that often omits entire native call paths from your profiles, so the data is often a lie)
• chrome://tracing/ (You can instrument your JS to show up here via console.time/console.timeEnd)