select all checkboxes using javascript?

hi,
    i am taking one datagrid in that grid i am taking checkboxes and out side of the datagrid i am taking 2 html buttons i.e., Select All and Select None.
 
     if i am click on select all button then all check boxes are selected and if i click select none button all checkboxes are unchecked
 
how to write javascript function todo this. please give a sample code for this.
 
Thanks 

Hi ajay raju,

Following is your solution and please mark my answer as accepted if it helped you:)

Step1: Call "MarkUnmarkMessages(true);" function on click of select all button and call    "MarkUnmarkMessages(false);" function on click of select none.

Step2: Add following script.

  function MarkUnmarkMessages(ischeck)
 {
           var grid = document.getElementById(<%=yourgrid.ClientID%>);
            var Msg = grid.getElementsByTagName('input');
            for (var m = 0; m < Msg.length; m++)
            {
                if (Msg[m].type == 'checkbox')
               {
                    Msg[m].checked = ischeck;
                  
                }
            }

           }

Thank you Maninder. your code is very helpful for me.