JavaScript has a foreach-style loop (for (x in a)), but it is extremely bad coding practice to use it on an Array. Basically, the array.length approach is correct. There is also a a.forEach(fn) method in newer JavaScripts you can use, but it is not guaranteed to be present in all browsers – and it’s slower than the array.length way.
EDIT 2017: “We’ll see how it goes”, indeed. In most engines now, .forEach() is now as fast or faster than for(;;), as long as the function is inline, i.e. arr.forEach(function() { ... }) is fast, foo = function() { ... }; arr.forEach(foo) might not be. One might think that the two should be identical, but the first is easier for the compiler to optimise than the second.
Belated EDIT 2020: There is now for (const item of iterable), which solves the downsides of using for (item in iterable).