select all checkboxes using javascript?

Question

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