Consider this small code sample in Javascript. var arrayOfNumbers=[1,2,3,4,5,6,78]; for(var index in arrayOfNumbers){ console.log(index+1); }; Output of this snippet will be 01 11 21 31 41 51 61 Now this is because Arrays are treated as Objects in Javascript, so index is iterating over keys in an object . In this case the object will be represented as follows arrayOfNumbers={ "0":1, "1":2, "2":3, "3":4, "4":5, "5":6, "6":78}