JavaScript实现HTML复选框全选与反选功能

<html>
 <head>
 <title>html+css+js实现复选框全选与反选</title>
 <meta http-equiv="content-type" content="text/html;charset=utf-8" />
 <meta name="keywords" content="js,笔试题目" />
 <style type="text/css">
  table,tr,td
  {
  border:1px solid red;
  }
 </style>
 <script type="text/javascript">
  function quanxuan()
  {
  for(var i=1;i<=3;i++)
  {
   var cbox_id="cb"+i;
   var cbox=document.getElementById(cbox_id);
   //alert(cbox.value);
   if(document.getElementById("cb_quanxuan").checked)
   cbox.checked=true;
   else
   cbox.checked=false;;
  }
  }
  function fanxuan()
  {
  for(var i=1;i<=3;i++)
  {
   var cbox_id="cb"+i;
   var cbox=document.getElementById(cbox_id);
   if(document.getElementById("cb_fanxuan").checked)
   {//选中反选框
   if(cbox.checked)
    cbox.checked=false;
   else
    cbox.checked=true;
   }
   else
   {
   if(cbox.checked)
    cbox.checked=false;
   else
    cbox.checked=true;
   }
  }
  }
 </script>
 
 </head>
 <body>
 <form id="form1">
  <table>
  <tr>
   <td><input type="checkbox" id="cb_quanxuan" onclick="quanxuan()" />全选</td>
   <td>复选框全选案例</td>
   <td>&nbsp;</td>
   <td>&nbsp;</td>
  </tr>
  
  <tr>
   <td><input type="checkbox" id="cb1" value="1" />1</td>
   <td>我是傻逼1</td>
   <td>&nbsp;</td>
   <td>&nbsp;</td>
  </tr>
  
  <tr>
   <td><input type="checkbox" id="cb2" value="2" />2</td>
   <td>我是傻逼2</td>
   <td>&nbsp;</td>
   <td>&nbsp;</td>
  </tr>
  
  <tr>
   <td><input type="checkbox" id="cb3" value="3" />3</td>
   <td>我是傻逼3</td>
   <td>&nbsp;</td>
   <td>&nbsp;</td>
  </tr>
  
  <tr>
   <td><input type="checkbox" id="cb_fanxuan" onclick="fanxuan()" />反选</td>
   <td>反选案例</td>
   <td>&nbsp;</td>
   <td>&nbsp;</td>
  </tr>
  </table>
 </form>
 </body>
</html>