js的onchange事件

<!DOCTYPE html>
<html>
<head>
    <meta charset="utf-8">
    <title></title>
</head>
<body>
    <select id="city">
        <option value="beijing">北京</option>
        <option value="shanghai">上海</option>
        <option value="nanjing">南京</option>
    </select>
    <select id="area">
        <option >海淀</option>
        <option >朝阳</option>
        <option >东城</option>
    </select>
</body>
    <script type="text/javascript">
        var select=document.getElementById("city"); select.onchange=function(){ var optionVal=select.value; switch(optionVal){ case 'beijing': var area=document.getElementById("area"); area.innerHTML="<option>海淀</option><option>朝阳</option><option>东城</option>"; break; case 'shanghai': var area=document.getElementById("area"); area.innerHTML="<option>浦东</option><option>徐汇</option><option>闸北</option>"; break; case 'nanjing': var area=document.getElementById("area"); area.innerHTML="<option>鼓楼</option><option>玄武</option><option>浦口</option>"; break; default: alert("error"); } }; </script>
</html>
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