as per your giving solution

JavaScript not working on ASP.Net DropDownList change inside UpdatePanel

it is working fine on event change on control, but Button behind script is not working. means that if else condition script is not displaying on browser after button event click.

<%@ Page Title="" Language="C#" MasterPageFile="~/Site.Master" AutoEventWireup="true" CodeBehind="test.aspx.cs" Inherits="Poltary.test" %>
<asp:Content ID="Content1" ContentPlaceHolderID="MainContent" runat="server">
    <asp:UpdatePanel ID="UpdatePanel1" runat="server">
    <ContentTemplate>
        <asp:DropDownList ID="ddlpurchase" runat="server" AutoPostBack="true"
            CssClass="form-control js-example-placeholder-single" Height="85px"
            ToolTip="Select " Width="280px">
            <asp:ListItem Value="0">---Select---</asp:ListItem>
            <asp:ListItem Value="1">ASPSnippets</asp:ListItem>
            <asp:ListItem Value="2">AspForums</asp:ListItem>
        </asp:DropDownList>
        <br />
        <asp:Button ID="btnRefresh" runat="server" Text="Refresh" OnClick="btnRefresh_Click" />
    </ContentTemplate>
</asp:UpdatePanel>

<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js"></script>
<script type="text/javascript" src="https://cdnjs.cloudflare.com/ajax/libs/select2/4.0.3/js/select2.min.js"></script>
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/select2/4.0.3/css/select2.min.css" />
<link href="http://ajax.aspnetcdn.com/ajax/jquery.ui/1.8.9/themes/start/jquery-ui.css"
    rel="stylesheet" type="text/css" />
<script type="text/javascript">
    $(function () {
        $(".js-example-placeholder-single").select2({
            placeholder: "Select",
            allowClear: true
        });
    });
             
    var prm = Sys.WebForms.PageRequestManager.getInstance();
    if (prm != null) {
        prm.add_endRequest(function (sender, e) {
            if (sender._postBackSettings.panelsToUpdate != null) {
                $(".js-example-placeholder-single").select2({
                    placeholder: "Select",
                    allowClear: true
                });
            }
        });
    };
</script>
</asp:Content>

C#

protected void btnRefresh_Click(object sender, EventArgs e)
{
    if (ddlpurchase.SelectedValue=="0")
    {
        Response.Write("<script>alert('Please Select Purchase')</script>");
    }
    else
    {

    }
}