Update index.js

Array.forEach will only iterate over keys that exist. So for example, forEach will only iterate twice for `[0,,,,,5]`, on elements 0 and 5. Likewise, take 
    
    var arr = [0,1,2]; 
    delete arr[1]; 
    arr.forEach(function(e){console.log(e);})

The console output will be 0 and 2.

If you were to check if a value is undefined, you run into the same problem because you can have an array element that exists with an undefined value. forEach should iterate over that, but not over a nonexistent element.

Using Object.keys solves this problem.