Delay the manipulation of a input value for predefined length of time

delay.js

var delay = (function(){
  var timer = 0;
  return function(callback, ms){
	clearTimeout (timer);
	timer = setTimeout(callback, ms);
  };
}());

$('.basket-quantity').keyup(function() {
	var $that = $(this);
	var quantity = $that.val();
	
	if(!isNaN(quantity) && quantity > 0 && quantity < 500) { 
		delay(function(){
			// do some kind of calculation on the quantity									
		}, 1500);
	}
});