There are several ASP.NET ServerControls that can use JavaScript functions instead of server side code-behind that posts the whole page back. ASP.NET 2.0 ServerControls such as Button, ImageButton, and LinkButton, have a property called OnClientClick, where you can specify JavaScript to execute when it is clicked. In ASP.NET 1.1, you will have to add a client side onclick event programmatically, to achieve the same results.

Adding "return false;" to the executed JavaScript will stop the page from doing a full postback after the client code is executed.

ASP.NET 2.0

<!-- HTML/JavaScript -->
<asp:ImageButton ID="ImageButton1" runat="server" OnClientClick="RunClientCode(); return false;" />
<script language="javascript" type="text/javascript">
  
   function RunClientCode(){
      alert('Client code executed');
   }

</script>

ASP.NET 1.1

// Code-behind
private void Page_Load(object sender, System.EventArgs e)
{
   this.ImageButton1.Attributes["onclick"] = "RunClientCode(); return false;";
}


<!-- HTML/JavaScript -->
<asp:ImageButton ID="ImageButton1" runat="server"  />
<script language="javascript" type="text/javascript">
  
   function RunClientCode(){
      alert('Client code executed');
   }

</script>

Original Article:
Q10171 - HOWTO: Using ImageButton controls to execute JavaScript instead of posting back