function incrementQty(id) {
      var value=id.value
      value = isNaN(value) ? 1 : value;
      value++;
      id.value = value;

}

function decrementQty(id) {
  var value=id.value
       value = isNaN(value) ? 1 : value;
  value > 1 ? value-- : value;
      id.value = value;
}