javascript中的条件下拉菜单，选择器出现问题

Question

我有一个条件下拉菜单，它可以工作，但这取决于我把它放在页面上的位置。如果我不把它放在一个特定的位置，菜单中的位置就根本不会填满。问题还在于，我想添加第二个条件下拉列表，无论我把它放在哪里，这个下拉列表都不起作用。所以我的问题是：我需要在页面上添加两次脚本来对应我的两个表单吗？事实上，我希望两个下拉列表都用完全相同的单词填充。

document.forms[0]['List'+i].length = 1;
document.forms[0]['List'+i].selectedIndex = 0;

我猜是和[0]有关。我尝试在表单中添加一个id，然后编写第二个脚本document.forms('myId')[]，但也不起作用。我该怎么做？

<script type="text/javascript">
var categories = [];
categories["startList"] = ["Programming","Science","History","Business and Economics","Software","Languages","Do it Yourself","Others"];
categories["Programming"] = ["Java","C++","C.","Python","Html","Php","Mysql","ObjectiveC","Android","Others"];
categories["Science"] = ["Mathematics","Physics","Biology","Chemistry","Medicine","Astronomy","Statistics","Others"];
categories["Others"] = ["All"] 
var nLists = 2; // number of lists in the set
function fillSelect(currCat,currList){
    var step = Number(currList.name.replace(/D/g,""));
    for (i=step; i<nLists+1; i++) {
        document.forms[0]['List'+i].length = 1;
        document.forms[0]['List'+i].selectedIndex = 0;
    }
    var nCat = categories[currCat];
    for (each in nCat)  {
        var nOption = document.createElement('option'); 
        var nData = document.createTextNode(nCat[each]); 
        nOption.setAttribute('value',nCat[each]); 
        nOption.appendChild(nData); 
        currList.appendChild(nOption); 
    } 
}
function getValue( L2, L1) {
    $.post( "", { List1: L1, List2: L2 } );
}
function init() {
    fillSelect('startList',document.forms[0]['List1'])
}
navigator.appName == "Microsoft Internet Explorer" ? attachEvent('onload', init, false) : addEventListener('load', init, false);    
</script>


<form action="" method="post">
<select name='List1' onchange="fillSelect(this.value,this.form['List2'])">
<option selected>Category</option>
</select>
&nbsp;
<select name='List2' onchange="getValue(this.value,this.form['List1'].value)">
<option selected >Subcategory</option>
</select>
<input type="Submit">

Answer 1

修复了它。我只需要在那里添加一个选择器或1来对应我的第二个表单。

 document.forms[1]['List'+i].length = 1;
            document.forms[1]['List'+i].selectedIndex = 0;
        }
        var nCat = categories[currCat];
        for (each in nCat)  {
            var nOption = document.createElement('option'); 
            var nData = document.createTextNode(nCat[each]); 
            nOption.setAttribute('value',nCat[each]); 
            nOption.appendChild(nData); 
            currList.appendChild(nOption); 
        } 
    }
    function getValue( L2, L1) {
        $.post( "", { List1: L1, List2: L2 } );
    }
    function init() {
        fillSelect('startList',document.forms[1]['List1'])