gistfile1.js

addLoadEvent(function() {
  var dropdowns = form.getElementsByTagName("select");
    for(var i = 0; i < dropdowns.length; ++i) {
      dropdowns[i].onchange = (function() {
        var dropdown = dropdowns[i];
        return function() {
          if(dropdown.value != "") {
            if(document.getElementsByClassName(dropdown.id).length == 0) {
              var input = document.createElement("input");
              input.type = "hidden";
              input.className = dropdown.id;
              input.name = "lists[]";
              input.value = "108";
              form.appendChild(input);
            }
          } else {
            var input = document.getElementsByClassName(dropdown.id);
            // TODO: There be another problem here --------^
            if(input.length != 0) {
              input[0].parentNode.removeChild(input[0]);
            }
          }
        }
      })();
    }
  });