Does Javascript compile or two-pass interpret?

Question

I'm an admitted novice JavaScript programmer and am attempting to learn more. So I turn to you folks for help, with this easy question :). The O'Reilly book that I'm reading keeps referring to the compile-time of the JavaScript code. My knowledge of functional programming (scheme and the likes) tells me that the JavaScript is actually interpreted by the browser, most likely requiring two passes through the JavaScript.

Am I incorrect in my assessment? Or is the compile-time that the book references actually just the first pass of the interpreter, similar to how Perl or Python would function? Thanks!

Answer 1

It is browser-dependent. Look up WebKit's SquirrelFish Extreme and Google V8 to see what's at the fastest end of things, and look at Mozilla's JaegerMonkey for that implementation.

AFIAK V8 and SFX are JITs, so they compile JS code to native. JaegerMonkey and TraceMonkey combine in Firefox to form a system where if code would be faster traced, TraceMonkey executes it, and if code were faster native, JaegerMonkey compiles it, just like SFX.