如何实现checkbox的全选/取消

 

 <script language = "Javascript">
    function selectAll(){
      var len = document.form1.elements.length;
     for (var i=0;i<len;i++){
       var ob = document.form1.elements[i];
       if (ob.name == "checkbox"){
        ob.checked = document.form1.chkall.checked;
       }
     }
    }
  </script> 

 <input type="checkbox" name="chkall" id="chkall"  onclick="selectAll();"><font size = "2">全选/不选</font>