I created ASP.NET user control with javascript function :
<%@ Control Language=\"C#\" AutoEventWireup=\"true\" CodeBehind=\"TestControl.ascx.cs\" Inherits
You need to register your scripts with ClientScriptManager - this way they can be registered once, regardless of how often the control has been added to the page:
// Get a ClientScriptManager reference from the Page class.
ClientScriptManager cs = Page.ClientScript;
// Check to see if the startup script is already registered.
if (!cs.IsStartupScriptRegistered(cstype, csname1))
{
String cstext1 = "alert('Hello World');";
cs.RegisterStartupScript(cstype, csname1, cstext1, true);
}