checkbox value

function checkset()
{
 var boxObj = document.getElementsByName("checkbox");
 for(i=0;i<boxObj.length;i++)
 {
  if(boxObj[i].checked)
  {
     document.getElementById("ctl00_ContentPlaceHolder1_hid").value = boxObj[i].value
     var b= document.getElementById("ctl00_ContentPlaceHolder1_hid").value;
     alert(b);
  }
 }
}