Register and Call JavaScript Function From Code Behind in Asp.Net C# and Vb

Last Updated: 14th March 2023

We know what is JavaScript and why it is used. JS 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.

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 button, which when clicked will call a JS function registered using Asp.Net.-->
            <input type="button" value="Click it" onclick="startup()" />
        </div>
    </form>
</body>
<script>
    // This is the script, which 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.

Note: 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

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