need javascript code to confirm deletion by buttonfield delete

This is the simplest possible way: http://www.w3schools.com/jsref/met_win_confirm.asp[^].

Alternatively, you can create wonderful and very flexible confirmation dialogs using jQuery. Check it out:
http://myclabs.github.io/jquery.confirm/[^].

If you need to learn jQuery (highly recommended), please see:
http://en.wikipedia.org/wiki/JQuery[^],
http://jquery.com/[^],
http://learn.jquery.com/[^],
http://learn.jquery.com/using-jquery-core/[^],
http://learn.jquery.com/about-jquery/how-jquery-works/[^] (start from here).

Your question about "onclienclick" looks irrelevant to me, I cannot even understand your idea. You need to get confirmation on the client side anyway. You don't have to use ASP.NET code behind to add an event handler, you generate some HTML, and it can contain any JavaScript.

[EDIT]

In response to the OP's comment to this answers:

The answer is in this comment. Note that his event can be cancelled:
http://msdn.microsoft.com/en-us/library/system.web.ui.webcontrols.gridviewdeleteeventargs.aspx[^],
http://msdn.microsoft.com/en-us/library/system.componentmodel.canceleventargs.cancel.aspx[^].

—SA

<asp:imagebutton id="ImageButton1" runat="server">
             ImageUrl ="~/images/grdbtndelete.png" CommandName ="Delete" 
OnClientClick="return confirm("Are you sure you want to delete this entry?");"  />
</asp:imagebutton>