Change key in js associative array

If I have:

var myArray = new Array();

myArray['hello'] = value;

How can I change the key 'hello' to something else?

Something like this would work.

var from = 'hello',
    to   = 'world',
    i, value = myArray[from];

for( i in myArray )
  if( i == from ) myArray.splice( i, 1 );

myArray[to] = value;

But is there a native function or a better way to do it?

edit:

Due to the lack of associative arrays in js, what I want to do modify the property name of an object as efficiently as possible.