How to Register and Call JavaScript Functions from Code Behind in ASP.NET (C# and VB)

Last Updated: 14th March 2023

JavaScript functions can be registered and called dynamically from server side applications like Asp.Net. Here in this article I'll show you how to call a JavaScript function using a code behind procedure in Asp.Net (C# and VB).

I am assuming that you have basic knowledge of JavaScript Statements and Code blocks.

The examples that I am going to show you here, covers three very popular Asp.Net code behind procedures to write Client Scripts.

The code behind procedures that I will be using in the example are,

01) RegisterClientScriptBlock – Register a block of Script without the "<script>" tags.

02) IsStartupScriptRegistered – Returns a Boolean (true or false), when a particular start up script is registered.

03) RegisterStartupScript – Register the Client Script from Code Behind.

The Script

<body>
    <form id="form1" runat="server">
        <div>
            <!--In-addition, I have a button control, which when clicked will call a JS function registered using Asp.Net.-->
            <input type="button" value="Click it" onclick="startup()" />
        </div>
    </form>
</body>
<script>
    // The script that will be called from code behind when the page loads.
    function script_CalledFrom_CodeBehind(servertime) {
        alert('Current Server Time: ' + servertime);        // Alert server time.
    }
</script>

The JavaScript function that I am going to "register and call" using code behind procedure, is the one I have declared inside the <script> tag in the header section. The function is script_CalledFrom_CodeBehind().

In addition, I have added a Button control in the body section. The button’s "click event" will call another function named "startup()". If you look carefully the markup and script, I have not declared the "startup()" function anywhere yet. In fact, I'll register the function using code behind procedure.

👉 The fact is, I am going to call and execute two JavaScript functions. The first script is at the client side, and the other script I have registered at the server side of the application.

Code Behind (C#)

Now see the code behind procedure.

using System;

public partial class _Default : System.Web.UI.Page 
{
    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            // This is a first JS script, which will be executed when the page loads.
            Page.ClientScript.RegisterClientScriptBlock(this.GetType(), 
                "Server Time", "window.onload = function () { script_CalledFrom_CodeBehind('" + 
                DateTime.Now.TimeOfDay + "'); }", true);

            // This is the second JS script, which will be called when the button (see the markup) is clicked.
            if (!(Page.ClientScript.IsStartupScriptRegistered("Registered Script")))    First, check if script is registered.
            {
                string sScript = "";
                sScript = sScript + "function startup() {";
                sScript = sScript + "alert('Calling another script on Button Click event." + 
                    "This script is registered at Code Behind.');";
                sScript = sScript + "}";

                // Register now.
                Page.ClientScript.RegisterStartupScript(this.GetType(), "Registered Script", sScript, true);
            }
        }
    }
}

Option Explicit On

Partial Class _Default
    Inherits System.Web.UI.Page

    Protected Sub form1_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles form1.Load
        If Not IsPostBack Then

            ' This is a first JS script, which will be executed when the page loads.
            Page.ClientScript.RegisterClientScriptBlock _
                (Me.GetType, "Server Time", "window.onload = function () {script_CalledFrom_CodeBehind('" & TimeOfDay & "');}", True)

            ' This is the second JS script, which will be called when the button (see the markup) is clicked.
            If Not (Page.ClientScript.IsStartupScriptRegistered("Registered Script")) Then
                Dim sScript As String = ""
                sScript = sScript & "function startup() {"
                sScript = sScript & "alert('Calling another script on Button Click event." & _
                    "This script is registered at Code Behind.');"
                sScript = sScript & "}"

                ' Now register.
                Page.ClientScript.RegisterStartupScript(Me.GetType, "Registered Script", sScript, True)
            End If
        End If
    End Sub
End Class

← PreviousNext →