var keys = Object.keys(obj),
length = keys.length,
key, i;
for (i = 0; i < length; i++) {
key = keys[i];
}
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.
Object.keys(obj).forEach(function (key) {
console.log(obj[key]);
});
But note that in your specific case chances are the cost of a bunch of console.log() calls completely swamps the cost of either a for loop or a forEach call. console.log() has _incredibly_ complicated behavior.
Deleting properties is guaranteed to not break iteration and deleting an as-of-yet unvisited property causes it to never be visited.