代码如下：
<script> var OptLstTxt = new Array; var OptLstVal = new Array; var OptLen = 0; function NoDupl(SelObjFrom, SelObjTo) { var OldToVal = SelObjTo.options[SelObjTo.selectedIndex].value; if (OptLen == 0) { OptLen = SelObjFrom.length; for (var i = 1; i < OptLen; i++) { OptLstTxt[i] = SelObjFrom.options[i].text; OptLstVal[i] = SelObjFrom.options[i].value; } } var j = 1; for (var i = 1; i < OptLen; i++) { if (OptLstVal[i] != SelObjFrom.options[SelObjFrom.selectedIndex].value) { if (j == SelObjTo.length) { SelObjTo.options[j] = new Option(OptLstTxt[i]); } else { SelObjTo.options[j].text = OptLstTxt[i]; } SelObjTo.options[j].value = OptLstVal[i]; if (OptLstVal[i] == OldToVal) { SelObjTo.selectedIndex = j; } j++; } } if (SelObjTo.length > j) SelObjTo.options[(SelObjTo.length - 1)] = null; } </script> <form method="POST" name="MForm"> <div align="center">下列四个选项中试试能不能选择重复的: <br> <select name="Color_1" onChange="NoDupl(this,document.MForm.Color_2)"> <option selected value=''>请选择你喜欢的颜色</option> <option value='R'>Red</option> <option value='J'>Yellow</option> <option value='G'>Green</option> <option value='B'>Blue</option> </select> <select name="Color_2" onChange="NoDupl(this,document.MForm.Color_1)"> <option selected value=''>请选择你喜欢的颜色</option> <option value='R'>Red</option> <option value='J'>Yellow</option> <option value='G'>Green</option> <option value='B'>Blue</option> </select> </div> </form>