关于javascript中全选和全不选的设置@慕课网原创_慕课网

<!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(i=0;i<hobby.length;i++)
           {
              if(hobby[i].type=="checkbox"){
                  hobby[i].checked=true;
              } 
           }

           }

        function clearall(){
            var hobby = document.getElementsByName("hobby");
            for(i=0;i<hobby.length;i++)
            {
                hobby[i].checked=false;
            }
         // 任务2    

        }

        function checkone(){
            var j=document.getElementById("wb").value;
            var hobby=document.getElementById("hobby"+j);
            hobby.checked=true;
         // 任务3

        }

        </script>
    </body>
</html>