function checkone(){
var j=document.getElementById("wb").value;
var jj=parseInt(j); //把string转化为int
if(jj>=1&&jj<=6){
var hobby = document.getElementsByTagName("input");
hobby[jj-1].checked=true;
}
}
getElementsByTagName("input");选取的是所有input标签，包括按钮。你要判断的是点击按钮，其中一个复选框选中，这个地方要用getElementsByName("hobby").
<!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");
for(var i=0;i<hobby.length;i++){
if(hobby[i].type == "checkbox"){
hobby[i].checked = true;
}
}
// 任务1
}
function clearall(){
var hobby = document.getElementsByName("hobby");
for(var i=0;i<hobby.length;i++){
hobby[i].checked = false;
}
// 任务2
}
function checkone(){
var j=document.getElementById("wb").value;
document.getElementById("hobby"+j).checked = true;
// 任务3
}
</script>
</body>
</html>