Question

In the following code why the variable ‘a’ refer to the index rather than the value ?

 for  (var a in  Values) {

     alert(Values[a]);
 }

Answer 1

Think of a JavaScript Array as a normal Object with a special property named length (actually, it a bit more complex). So the for..in loop behaviour is identical as for other objects:

var a = new Array();

a[1] = "a";
alert(a.length); // 2
alert(a[0]); // undefined

a[1000] = "b"
alert(a.length); // 1001


a[-1] = "c";
alert(a[-1]); // c

a.abc="why not";

for(var key in a)
{
  alert(key+"="+a[key]);
}
// 1=a
// 1000=b
// -1=c
// abc=why not

Also note that you can have gaps within your array without having to pay the memory price.