What's the cleanest way to write a non-blocking for loop in javascript?

So, I've been thinking about a brain teaser - what if I had a large object I for some reason had to iterate through in node js, and didn't want to block the event loop while I was doing that?

Here's an off-the-top-of-my-head example, I'm sure it can be much cleaner:

var forin = function(obj,callback){
    var keys = Object.keys(obj),
        index = 0,
        interval = setInterval(function(){
            if(index < keys.length){
                callback(keys[index],obj[keys[index]],obj);
            } else {
                clearInterval(interval);
            }
            index ++;
        },0);
}

While I'm sure there are other reasons for it being messy, this will execute slower than a regular for loop, because setInterval 0 doesn't actually execute every 0 ms, but I'm not sure how to make a loop with the much faster process.nextTick.

In my tests, I found this example takes 7 ms to run, as opposed to a native for loop (with hasOwnProperty() checks, logging the same info), which takes 4 ms.

So, what's the cleanest/fastest way to write this same code using node.js?