A better approach is to store the old value using .data. This spares the creation of a global var which you should stay away from and keeps the information encapsulated within the element. A real world example as to why Global Vars are bad is documented here
e.g
<script>
//look no global needed:)
$(document).ready(function(){
// Get the initial value
var $el = $('#myInputElement');
$el.data('oldVal', $el.val() );
$el.change(function(){
//store new value
var $this = $(this);
var newValue = $this.data('newVal', $this.val());
})
.focus(function(){
// Get the value when input gains focus
var oldValue = $(this).data('oldVal');
});
});
</script>
<input id="myInputElement" type="text">