Strings as keys of array

for( var i = arr.length; i--; console.log( arr[ i ] ) );

This will only give you the numeric indices, of course, but you can still loop over both numeric indices and string keys of your array like this:

for (var x in arr) {
    console.log(x + ": " + arr[x]);
}
/* (console output):
     0: 0
     1: 1
     2: 2
     3: 3
     something: aught
*/

In javascript there are 2 type of arrays: standard arrays and associative arrays

• [ ] - standard array - 0 based integer indexes only
• { } - associative array - javascript objects where keys can be any strings

So when you define:

var arr = [ 0, 1, 2, 3 ];

you are defining a standard array where indexes can only be integers. When you do arr["something"] since something (which is what you use as index) is not an integer you are basically defining a property to the arr object (everything is object in javascript). But you are not adding an element to the standard array.