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How do I generate JavaScript during an AJAX callback (postback) and then execute it on the browser?
void AjaxHandler(object sender, EventArgs e)
{
// call to database
string jscript = "alert('wibble');";
ScriptManager.RegisterStartupScript(this, this.GetType(), "uniqueKey", jscript);
}

ScriptManager.RegisterClientScriptBlock()
Page.RegisterStartupScript()
Page.RegisterClientScriptBlock()
Page.ClientScript.RegisterStartupScript()
Page.ClientScript.RegisterClientScriptBlock()

ScriptManager.RegisterStartupScript(this, this.GetType(), "uniqueKey", jscript, true);

<asp:ScriptManager runat="server" ID="scriptManager">
</asp:ScriptManager>
<script type="text/javascript">

    function endRequestHandler(sender, args) {
        var dataItems = args.get_dataItems();
        for(var key in dataItems){
            if(/^javascript:/.test(dataItems[key])){
                eval(dataItems[key].substring("javascript:".length));
            }
        }
    }
    Sys.WebForms.PageRequestManager.getInstance().add_endRequest(endRequestHandler);

</script>

<asp:UpdatePanel runat="server" ID="pnl">
    <ContentTemplate>
        <asp:Button runat="server" ID="btnClick" Text="Click me!" OnClick="btnClick_Click" />
    </ContentTemplate>
</asp:UpdatePanel>

protected void btnClick_Click(object sender, EventArgs e)
{
    ScriptManager.GetCurrent(Page).RegisterDataItem(this, "javascript:alert('hello world!');");
}