怎么会是"undefined"?

document.getElementsByName("myList")返回的是一个数组var lists=document.getElementsByName("myList");
if(lists.length>0){
document.writeln(lists[0].value);
}

<script language="JavaScript">
function a(){
var ListValue=document.getElementsByName("myList")[0].value;//select对象选中的值
document.writeln(ListValue);
}
</script>
document.getElementsByName("myList")[0].selectedIndex返回所选中元素的索引数
document.getElementsByName("myList")[0].value
document.getElementsByName("myList")[0].value
var ListValue=document.getElementsByName("myList").value=======>>var List=document.getElementsByName("myList")
var ListValue=List.options[List.selectedIndex].value
<html>
<head>
<script language="JavaScript">
function a(){
var ListValue=document.getElementById("myList").value
document.writeln(ListValue);
}
</script>
</head><body>
<select id="myList">
<option value="34">天f好 --> 13首</option>
<option value="35">天fg下大adf好 --> 12首</option>
<option value="38">天下大gfdadf --> 18首</option>
<option value="43">天下dfadf好 --> 9首</option>
</select><p onclick="a()">abc</p></body>
</html>
<script language="JavaScript">
function a(){
var ListValue=document.getElementsByName("myList")[0].value;//select对象选中的值
document.writeln(ListValue);
}
</script>
document.getElementsByName("myList")[0].selectedIndex返回所选中元素的索引数