请问加下划线这段for循环有什么问题么？为什么只能执行一次
<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312">
<title>无标题文档</title>
</head>
<body>
<form>
请选择你爱好:<br>
<input type="checkbox" name="hobby" id="hobby1"> 音乐
<input type="checkbox" name="hobby" id="hobby2"> 登山
<input type="checkbox" name="hobby" id="hobby3"> 游泳
<input type="checkbox" name="hobby" id="hobby4"> 阅读
<input type="checkbox" name="hobby" id="hobby5"> 打球
<input type="checkbox" name="hobby" id="hobby6"> 跑步 <br>
<input type="button" value = "全选" onclick = "checkall()">
<input type="button" value = "全不选" onclick = "clearall()">
<p>请输入您要选择爱好的序号，序号为1-6:</p>
<input id="wb" name="wb" type="text" >
<input name="ok" type="button" value="确定" onclick = "checkone()">
</form>
<script type="text/javascript">
function checkall()
{
var hobby = document.getElementsByTagName("input");
// 任务1 加下划线的这段如果吧注释的那句取消注释就只能执行一次
for(var i=0;i<hobby.length;i++)
{
//document.write(i);
if(hobby[i].type=="checkbox")
{
hobby[i].checked=true;
}
}
}
function clearall(){
var hobby = document.getElementsByName("hobby");
// 任务2
for(var i=0;i<hobby.length;i++)
{
// document.write(i);
if(hobby[i].type=="checkbox")
{
hobby[i].checked=false;
}
}
}
function checkone()
{
var j=parseInt(document.getElementById("wb").value);
var hobby = document.getElementsByName("hobby");
// 任务3
//document.write(j<hobby.length);
if(j<hobby.length){
for(var i=0;i<hobby.length;i++)
{
// document.write(i);
if(hobby[i].type=="checkbox")
{
hobby[i].checked=false;
}
}
hobby[(j-1)].checked=true;
}
else
{
alert('输入的数值溢出');
}
}
</script>
</body>
</html>
