<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;
               }
           }
        }
        function clearall(){
            var hobby = document.getElementsByName("hobby");   
             for(var i=0;i<hobby.length;i++){
                   hobby[i].checked=false;
        }
        
        function checkone(){
            var j=document.getElementById("wb").value;
            var hobby=document.getElementById("hobby"+j);
            hobby.checked=true;
           
        }
以老夫30年代码观写经验来看,眼睛随便一瞟 就看见你这里少写一个结束的  }
就看见你这里少写一个结束的  }  
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>Document</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++)
{
if(hobby[i].type=='checkbox')
{
hobby[i].checked=false;
}
}
// 任务2
}
function checkone(){
var j=document.getElementById("wb").value;
var hobby=document.getElementsByName('hobby');
for(var i=0; i<j.length; i++)
{
hobby[(j.charAt(i))-1].checked=true;
}
// 任务3
}
</script>
</body>
</html>