Get an array of keys for the given array. The returned array has no keys, has the same indexes as the given array and instead of values has the keys(as string) from the initial array. If an element has no key, an null element will be added.
Return value:
Return an array containing the keys associated with the given array.
Example
array["test"]=0;
array["test2"]=1;
array[2]="some value"; // notice that element 3 has no key
...
var keys=GetKeys(array);
...
// will return keys[0]="test", keys[1]="test2", keys[2]=null.
// WARNING: keys[2] is not a string (is a number, null)