do-after-load.ascx

<%@ Control Language="C#" AutoEventWireup="true" CodeBehind="do-after-load.ascx.cs" Inherits="controls.DoAfterLoad" %>
<asp:UpdatePanel runat="server" UpdateMode="Always">
    <ContentTemplate>
        <asp:HiddenField runat="server" ID="DoAfterLoadJavaScriptHiddenField" />
    </ContentTemplate>
</asp:UpdatePanel>

do-after-load.ascx.cs

using System;
using System.Collections.Generic;
using System.Linq;

namespace MpFloors.controls
{
    public partial class DoAfterLoad : System.Web.UI.UserControl
    {
        private Dictionary<object, string> _scripts = new Dictionary<object, string>();
        //public string DoAfterLoadJavaScript { get; set; }

        public void AddDoAfterLoadJavaScript(object key, string script)
        {
            _scripts[key] = script;
        }

        public void Clear()
        {
            _scripts.Clear();
        }

        protected void Page_Load(object sender, EventArgs e)
        {
            Page.ClientScript.RegisterStartupScript(
                GetType(),
                "DoAfterLoadJavaScriptEngine",
                @"            
                Sys.WebForms.PageRequestManager.getInstance().add_pageLoaded(function(s, e) {  
                    $(""[id$='_DoAfterLoadJavaScriptHiddenField']"").each(function() {eval(this.value)}); 
                }); 
            ", true);
        }

        protected void Page_PreRender(object sender, EventArgs e)
        {
            DoAfterLoadJavaScriptHiddenField.Value = string.Join(";",
                _scripts.Where(item => !string.IsNullOrWhiteSpace(item.Value)).Select(item => item.Value.TrimEnd(';')));
        }

    }
}