How do I stop a page refresh when C# code behind is executed and a JavaScript startup script is registered?

Question

I have a button that executes code behind in c#. The code behind gets some information from the database and writes a fairly long piece of JavaScript that it then registers and runs on the client. The code behind looks something like this:

protected void Button1_Click(object sender, EventArgs e)
{
   System.Text.StringBuilder sb = new System.Text.StringBuilder();
   sb.Append(@"
        <script language='javascript'>
        //Lots of JavaScript
       </script>");
   System.Web.UI.ScriptManager.RegisterStartupScript(this, this.GetType(), "JCall1", sb.ToString(), false);
}

The code works and does what it should. The problem is between the press of the button and the execution of the JavaScript, I get a page reload flash. I would like to stop that flash. I can tell it is happening before the JavaScript executes because one of the first things I do in the JavaScript is start a processing icon. It spins while some promises are resolved/handled. The flash occurs before the icon starts processing. Once the JavaScript starts (icon appears and spins), everything is good. It is probably something I don't understand in the lifecycle of this button. I'm surmising that when I RegisterStartupScript, the page is refreshing to load the new script. I can't do it all in JavaScript Client-Side because internal data needs to be provided to the API's. Yes, it would be great if the site was.Net Core and I could do it all in a Razor Page but this is not what I am dealing with. It's WebForms. There may not be anything I can do about it but if there is, I could use some help.

Answer 1

I think what you're looking for is for the button click to execute asynchronously. This other SO post should help you craft it to your needs: Calling async method on button click

Answer 2

Yes, it would be great if the site was.Net Core and I could do it all in a Razor Page but this is not what I am dealing with.

Poorly placed sarcasm aside, the answer is the same in both a modern Asp.Net MVC Core application and in legacy WebForms: nothing you wrote here implies server-side code, so don't put it on the server side.

If you want to handle a button and make it execute some JavaScript code (ie, on the client, which is what your block is doing), simply register a handler on the client side and write your JS code there:

$('#<%= Button1.ClientID %>').click(function() {
    // code here
});

Because otherwise what you're doing is wrong on so many levels, not the least of which is the fact that ScriptManager.RegisterStartupScript works by inserting a <script> tag in the page itself, and it should be obvious to you that it requires reloading the entire page for this.