Js checkbox全选和全不选

[javascript]
function checkAllViewable(bool){ 
     var boxs = document.getElementsByName("viewables"); 
     for (var i = 0; i < boxs.length; i++) { 
         if (boxs[i].type == "checkbox")  
             boxs[i].checked = bool; 
     } 
 } 

[javascript]
<A href="javascript: checkAllViewable(true)">可见全选</A>