`for ... in` is a relatively slow construct anyway, the faster option (a lot more verbose) is:

    var keys = Object.keys(obj),
        length = keys.length,
        key, i;
    for (i = 0; i < length; i++) {
      key = keys[i];
    }

While this is not exactly the same as for..in, it usually behaves how you'd expect and is significantly faster for a couple of reasons:

1. In a for..in loop the engine must keep track of the keys already iterated over, whereas in the fast version we can simply increment a counter and do a fast array lookup.

2. It's possible to add properties to the object that you're iterating within the body of the for..in statement, and these will be iterated too. Doing such a thing is obviously very rare but it's the kind of edge case the JS engine must keep track of.