Javascript cycle "for (index in array)" is used for arrays - replace with normal "for (index = 0; i < array.length; i++)"

Currently in some places arrays are accessed as objects in cycles:
var arr = ['one', 'two'];
for (var elem in arr)

{ alert( arr[elem] ); }

This approach works until someone adds new methods to Array class declaration. I propose to change such approach to normal "for" cycle:
var arr = ['one', 'two'];
for (var elem = 0; i < arr.length; elem++) { alert( arr[elem] ); }