Is Javascript parsed/interpreted on load? (IE)

Question

I Know, for instance, that when Chrome downloads a Javascript file, it is interpreted and JITed.

My question is, when IE6,7,8, first download a Javascript file, is the entire thing parsed and interpreted?

My understanding was that only top level function signatures and anything executed in the global scope was parsed on load. And then function bodies and the rest were parsed on execution.

If they are fully parsed on load, what do you think the time savings would be on deferring the function bodies to be downloaded and parsed later?

Answer 1

Because defining a function is actually an operation, yes, your entire javascript file is parsed, and all of the top-level operations are interpreted. The code inside of your functions is not actually executed until it's called, but it is parsed.

for example:

var i=0;
var print = function( a ) {
  document.getElementById( 'foo' ).innerHtml = a;
}

Everything gets parsed in the above example, and lines 1 and 2 get executed. However, line 3 doesn't get executed until it's called.

There are little "perceptual games" you can play with your users, like putting the script tags at the bottom of the HTML instead of at the top, so that the browser will render the top of the page before it receives the instructions to download and parse the javascript. You could probably also push your function definitions into a document.onload function, so that they don't get executed until after the whole page is loaded and in memory. However, this could cause a "flash of unstyled content" if your javascript is applying visual styles to things (such as jQuery UI stuff).

Answer 2

They are fully parsed on load. (IE has to parse the script to know where each function body ends, of course.) In the open-source implementations, every function is compiled to bytecode or even to machine code at the same time, and I imagine IE works the same way.

If you have a page that's actually loading too slowly, and you can defer loading 100K of script that you're probably not going to use, it might help your load times. Or not—see the update below.

(Trivia: JS benchmarks like Sunspider generally do not measure the time it takes to parse and compile the code.)

UPDATE – Since I posted this answer, things have changed! Implementations still parse each script on load at least enough to detect any SyntaxErrors, as required by the standard. But they sometimes defer compiling functions until they are first called.