arr.splice(newIndex, 0, arr.splice(oldIndex, 1));
- You have to know the current index of the element you want to move and the new index you want to move it to.
- The inner call to arr.splice (with  on the end) pulls the element you want to move out of the array.
- The outer call to arr.splice inserts it back into the array at the new index
Excerpted from a post on stackoverflow.com