javascript : Select all checkbox

function checkall()
{
   var len =document.getElementsByName('asid');
   if(len.length != null)
   {
     for (var i = 0; i < len.length ; i++) {
   len[i].checked = true;
   }
   }
   else
   {
        len.checked = true;
   }
}

function clearall()
{

var len =document.getElementsByName('asid');
   if(len.length != null)
   {
     for (var i = 0; i < len.length ; i++) {
   len[i].checked = false;
   }
   }
}