JS 动态添加列表框项效果代码_表单特效

<table width="200" border="0" cellspacing="0"> <tr> <td colspan="3"><span id="selectmsg">*请选择1-3个知识点.</span></td> </tr> <tr> <td><select name="selectleft" size="10" id="selectleft"> <option value="1">1213123123</option> <option value="2">2123123123</option> <option value="3">3213123213</option> <option value="4">4123213213</option> <option value="5">5213123213</option> <option value="6">6213123213</option> </select></td> <td> <input type="button" id="addto" class="operateBtn" value=">>"> <input type="button" id="moveback" class="operateBtn" value="<<"> </td> <td><select name="selectright" size="10" id="selectright"> </select></td> </tr> </table> <script type="text/javascript"> var addTo = document.getElementById("addto"); var moveBack = document.getElementById("moveback"); var selectLeft = document.getElementById("selectleft"); var selectRight = document.getElementById("selectright"); var count = 0; addTo.onclick = addOption; moveBack.onclick = delOption; //这个函数检验传入的值是否在有边出现过！ function hasOption(str){ for(var i=0;i<document.getElementById("selectRight").options.length;i++){ if(document.getElementById("selectRight").options.value==str){ return false; } } return true; } function addOption(){ var nowIndex = document.getElementById("selectRight").options.length; //右边的下一个索引 var selectIndex = document.getElementById("selectLeft").options.selectedIndex; //做边被选种项索引 var selectText = document.getElementById("selectLeft").options[selectIndex].text; //被选种项文本 var selectValue = document.getElementById("selectLeft").options[selectIndex].value; //被选种项值 if(selectIndex!=-1){ if(!(hasOption(selectValue))){ //如果选了一项且右边没有，执行 if(count<3){ var newoption = new Option(selectText,selectValue,false,false); document.getElementById("selectRight").options[nowIndex] = newoption; count+=1; document.getElementById("selectmsg").innerHTML="*你还可以选择"+(3-count)+"个知识点"; }else document.getElementById("selectmsg").innerHTML="<font color=red>*温馨提示,你只能选择三项！"; }else document.getElementById("selectmsg").innerHTML="<font color=red>*温馨提示,不能重复选择！"; } } function delOption(){ var selectIndex = document.getElementById("selectRight").options.selectedIndex; if(selectIndex!=-1){ document.getElementById("selectRight").options[selectIndex] = null; //清空选种项 count-=1; document.getElementById("selectmsg").innerHTML="*你还可以选择"+(3-count)+"个知识点"; } } function hasOption(str){ var tmp=false; for(i=0;i<count;i++){ if(str==document.getElementById("selectRight").options[i].value){ tmp=true; break; } else tmp=false; } return tmp; } </script>
[Ctrl+A 全选 注:如需引入外部Js需刷新才能执行]