初学者，如何做一个选择级联

菜鸟求助，怎么做一个选择级联，
<html>
<title>考试申请</title>
<head>
<script type="text/javascript">

var termlist=new Array();
termlist['第一学期']=["c#","计算机基础","电子商务概论",];
termlist['第二学期']=["dreameaver","市场营销","网络","asp",];
function changeterm()
{
var term=document.getElementById("selterm").value;
var class=document.getElementById("selclass");
class.options.length=0;
for(var i in termlist)
{
if (i==term)
{
for(var j in termlist[i])
class.add(new option(termlist[i][j],null))

}
}

}

</script>
</head>
<body>
<select id="selterm" onChange="changeterm()" style="width:100px">
<option>选择学期</option>

</select>
<select id="selclass" style="width:100px">
<option>选择课程</option>
</select>

</body>
</html>

[解决办法]

HTML code

&lt;html&gt;&lt;title&gt;考试申请&lt;/title&gt;&lt;head&gt;&lt;/head&gt;&lt;body&gt;&lt;select id="selterm" onChange="changeterm()" style="width:100px"&gt;&lt;option&gt;选择学期&lt;/option&gt;&lt;/select&gt;&lt;select id="selclass" style="width:100px"&gt;&lt;option&gt;选择课程&lt;/option&gt;&lt;/select&gt;&lt;script type="text/javascript"&gt;    var termlist = [];    termlist['第一学期'] = ["c#","计算机基础","电子商务概论"];    termlist['第二学期'] = ["dreameaver","市场营销","网络","asp"];    var tmp = [];    var obj1 = document.getElementById('selterm');    var obj2 = document.getElementById('selclass');    for(var x in termlist){        obj1.add(new Option(x, null));        tmp.push(x);    }    function changeterm(){        var l = termlist[tmp[obj1.selectedIndex - 1]];        obj2.length = 1;        for(var i = 0, len = l.length; i&lt; len; i++){            obj2.add(new Option(l[i], null));        }    }            &lt;/script&gt;&lt;/body&gt;&lt;/html&gt;
[解决办法]
HTML code&lt;html&gt;&lt;title&gt;考试申请&lt;/title&gt;&lt;head&gt;&lt;script type="text/javascript"&gt;var termlist = new Array();termlist['第一学期'] = ["c#","计算机基础","电子商务概论"];termlist['第二学期'] = ["dreameaver","市场营销","网络","asp"];function changeterm() {    var term = document.getElementById('selterm').value;    var cls = document.getElementById('selclass'); //class是Javascript语言保留字，不能用作变量名    cls.options.length = 0;    for (var i in termlist) {        if (i == term) {            for(var j in termlist[i]) cls.options.add(new Option(termlist[i][j], termlist[i][j]));        }    }}window.onload = function() {    var obj = document.getElementById('selterm');    for(var i in termlist) obj.options.add(new Option(i, i));}&lt;/script&gt;&lt;/head&gt;&lt;body&gt;&lt;select id="selterm" onChange="changeterm()" style="width:100px"&gt;  &lt;option&gt;选择学期&lt;/option&gt;&lt;/select&gt;&lt;select id="selclass" style="width:100px"&gt;  &lt;option&gt;选择课程&lt;/option&gt;&lt;/select&gt;&lt;/body&gt;&lt;/html&gt;

初学者如何做一个选择级联

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