<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 
           for(var i=0;i<=hobby.length;i++){
               if(hobby[i].type=="checkbox"){
                   hobby[i].checked="checked";
               }
           }
        }
        function clearall(){
            var hobby = document.getElementsByName("hobby");
            
         // 任务2    
            for(var i=0;i<=hobby.length;i++){
                if(hobby[i].type=="checkbox"){
                    hobby[i].checked="";
                }
            }
        }
        
        function checkone(){
            var j=document.getElementById("wb").value;
        
         // 任务3
        if(0<j && j<7){
            check = document.getElementById("hobby"+j);
            check.checked="checked";
        }
        }
        
        </script>