大家好，我初学Javascript，这儿有一串代码，里面有一段看不懂，麻烦大家帮一下，先谢了
程序代码：<html>
<head>
<title>自定义对象示例</title>
<style type="text/css">
<!--
body {
font-family: "宋体";
line-height: 20px;
text-align: center;
background-color:#9acdcd;
}
table {
font-size: 12px;
text-align:center
}
.title {
color: #FFFFFF;
text-align: center;
font-family: "宋体";
font-size: 14px;
line-height: 25px;
font-weight: bold;
}
.blue {
color: blue;
font-size: 12px;
}
#Text1
{
width: 203px;
}
#Button1
{
width: 62px;
}
-->
</style>
</head>
<body>
<script type="text/javascript">
Computation=function(){};
Computation.prototype={
Factorial:function(num)
{
var rs=1;
for(i=1;i<=num;i++)
{
rs*=i;
}
return rs;
},
cubic:function(num)
{
var rs;
rs=Math.pow(num,3);
return rs;
}
}
function IsNum(num)
{
var reNum=/^\d*$/;
return(reNum.test(num));
}
function getResult(num)
{
if(IsNum(num))
{
var chk= document.getElementsByName('operation');
var op="";
for(var i = 0; i< chk.length; i++)
{
if(chk[i].checked)
{
op=chk[i].value
}
}
switch(op)
{
case "阶乘":
res=Computation.prototype.Factorial(num);
alert(num+"的阶乘为"+res);
break;
case "三次方":
res=Computation.prototype.cubic(num);
alert(num+"的三次方结果为"+res);
break;
}
}
else
alert(num+"不是数值类型");
}
</script>
<table style="width:100%;">
<tr>
<td style="font-size: 18px">
请输入操作数：<input id="Text1" type="text" /></td>
</tr>
<tr>
<td>
<input id="Radio1" name="operation" type="radio" value="阶乘" />阶乘
<input id="Radio2" name="operation" type="radio" value="三次方" />三次方</td>
</tr>
<tr>
<td>
<input id="Button1" type="button" value="确定" onclick="getResult(Text1.value)"
/></td>
</tr>
</table>
</body>
</html>
问题：这段for语句是什么意思，在这段代码中启了什么作用啊
for(var i = 0; i< chk.length; i++)
{
if(chk[i].checked)
{
op=chk[i].value
}
