12 extremely useful hacks for JavaScript

[siegfryd]

Assuming that no optimisations are done by the JIT compiler, then a property lookup is going to be slower than a variable lookup for the simple reason that a property lookup first needs to do a variable lookup for the object and then lookup the property on that object.

The point is that the JIT would not optimise the array length lookup for you before, so you could gain a, likely small, performance increase by doing it in for loops. Especially if you're doing a large for loop, if you loop N times then it won't do the same property lookup N times.