I'm writing a simple `for...in` loop in javascript, and wondering why the `key` is a string and not a number?

Why is it so, and can I change that to be a number?

```
var array = ["a", "b", "c"];

for (var key in array) {
   console.log(typeof key); //string
   console.log(key + 1); //expected output : 01, 11, 21...
}

```