Pre-parsing is an obvious big win, but there's also classical compiler optimizations: constant propagation, DCE, certain inlining, etc. Local variables can be statically determined to be non-captured, which means they can be stack allocated.
One totally bizarre but profoundly useful property of JS is the restrictions on eval. eval has the ability to run new code, but this code is globally scoped unless it's run as the "eval" function itself. [1] Thus it's easy to decide which local variables are immune to eval; this in turn unlocks lots of optimizations. To my knowledge this weirdo behavior is not present in Python or other dynamic languages.
In JS functions are first class, so one might attempt:
function wut() {
var x = 1;
var obj = {sneaky: eval};
obj.sneaky("x++");
console.log(x);
}
The answer is no: because the caller's name 'obj.sneaky' does not literally compare equal to "eval", the eval function is run with global scope instead of local scope. Therefore the 'x' inside the eval'd string is a global property, not the local variable.
So with this analysis we can (statically) apply constant propagation and replace x with 1.
Now if we replace obj.sneaky with eval, the string comparison succeeds, it becomes "direct eval", and the constant propagation is invalid.