Guesswork, static analysis, experiment etc. JSC does all kinds of black magic to make JavaScript run fast. For a large amount of programs (basically all "class

Guesswork, static analysis, experiment etc. JSC does all kinds of black magic to make JavaScript run fast. For a large amount of programs (basically all "classical" programs like you'd read about in the past rather than modern apps) you have a fairly clear path through the code for their data, this means that you can optimize that path. This doesn't mean the performance will be as good as a statically typed language but keep in mind 90% of compiler optimization is basically useless on many programs vs. the 10% that makes the important bits run fast.