Click to add or subtract to qty box

qtybox.js

      //product page qty
      $('.sp-plus').on('click', function(){
          var newVal = (parseInt($('.sp-input').val(),10) +1);
          var oldVal = parseInt($('.sp-input').val(), 10);
          if (isNaN(oldVal) === true){
              $('.sp-input').val(1);
          } else {
              $('.sp-input').val(newVal);
          }
      });

      $('.sp-minus').on('click', function(){
          var newVal = (parseInt($('.sp-input').val(),10) -1);
          var oldVal = parseInt($('.sp-input').val(), 10);
          if(oldVal <= 1) {
              return false;
          } else if (isNaN(oldVal) === true){
              $('.sp-input').val(1);
          } else {
              $('.sp-input').val(newVal);
          }
      });