Check all checkboxes through JQuery on a Checkbox event

 //chkAllSub function calls on sub checkboxes click
//chkAll function calls on main checkbox
//.sub_chk is the class of sub checkboxes
//master is the ID of main checkbox



function chkAllSub()
{
  $(".sub_chk").each(function(){
    var chkboxLength = $("input.sub_chk").length;
    if($("input.sub_chk:checked").length >= chkboxLength)
    {
      $("#master").attr('checked',true);
    }
    else
    {
      $("#master").attr('checked',false);
    }
  })
}
function chkAll()
{
  if($("#master").is(':checked',true))
  {
    $(".sub_chk").attr('checked',true);
  }
  else
  {
    $(".sub_chk").attr('checked',false);
  }
}