Uncaught TypeError: Cannot read property 'on' of null [duplicate]

Answer 1

In your site, you have both prototype.js and jQuery added. The $ here calls prototype.js instead of jQuery. So instead of that code, please change it to:

jQuery('.addtocart').on('click', function(){

  //get the value of the "data-value" attribute for that link
  var vehicleRegistration = jQuery(this).data('value');
  //save it to localStorage
  localStorage.setItem('vehicleRegistration', vehicleRegistration);


  //read from localStorage
  if( localStorage.getItem('vehicleRegistration') ){
    //add the value to the form input
    jQuery('#field34').val( localStorage.getItem('vehicleRegistration') );
  }

});

Or you can also do this:

(function ($) {
    $('.addtocart').on('click', function(){

      //get the value of the "data-value" attribute for that link
      var vehicleRegistration = $(this).data('value');
      //save it to localStorage
      localStorage.setItem('vehicleRegistration', vehicleRegistration);


      //read from localStorage
      if( localStorage.getItem('vehicleRegistration') ){
        //add the value to the form input
        $('#field34').val( localStorage.getItem('vehicleRegistration') );
      }

    });
})(jQuery);