How to change java script parameter from code behind

Question

I'm curious if there is possibility to change java script parameter from code behind if specific text location (TextBox19) have to be moved to TextBox20.

.aspx

    function myFunction()
        {
            var progValue1 = <%=ProgValue1%>;

            document.getElementById({ value: progValue1 }).value = "something";
        };

<asp:Button ID="Button2" runat="server" Text="Button" OnClientClick="myFunction()" OnClick="Button2_Click" />

<asp:Button ID="Button1" runat="server" Text="Change text location" OnClientClick="SomeMethod" />

.aspx.cs

    private string _progValue1 = "\"" + "<%=TextBox19.ClientID%>" + "\"";

    protected string ProgValue1 { get { return this._progValue1; } }

    private void SomeMethod(object sender, EventArgs e)
    {
        this._progValue1 = "\"" + "<%=TextBox20.ClientID%>" + "\"";
    }
    protected void Page_Load(object sender, EventArgs e)
    {
        //wywołanie skryptu js myFunction() 
        ClientScript.RegisterStartupScript(Page.GetType(), "OnLoad", "myFunction();", true);

}

There is possibiility to do it in this way?

Avrohom Yisroel · Accepted Answer · 2022-10-07 15:28:55Z

1

It sounds like you may be unclear as to what runs where and when.

The code in your 'apx.cs file runs just once (per request), on the server, and generates text (usually HTML, but can include JavaScript, CSS, etc). That is then sent to the client, and the server's job is over. This usually a web browser (but doesn't have to be).

After the generated text has been sent to the client, the server's task is over, and it does not have any more to do with what was sent out, unless the client then posts back, but that's basically a new request.

So, your first code snippet will only run when the page is first requested. After that, the only way to change code on the client is to use AJAX.

So I think the basic answer to your question is "no," but without knowing what you're actually trying to do, I can't really say more. It would help if you explain your requirements from the user's point of view, then we'd be able to comment more on how to do it.

Note that my comments refer to ASP.NET WebForms. If you use Blazor, then you can change the client code to your heart's delight!

answered Oct 7, 2022 at 15:28

Avrohom Yisroel

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3 Comments

Eninede Over a year ago

Post stackoverflow.com/questions/11756838/… is suggesting possibility of changing javascript value by using code behind. Why would not have possibility to change a location (textbox19 -> textbox20) for this value in the same way?

Avrohom Yisroel Over a year ago

You can do that, which is basically what I was explaining in my answer. What you need to understand though is that with WebForms, the only time you can do this is when the page is rendered, ie on first load, or on a postback (which is really a first load, but where the client sends back some state that makes it look like it's a follow-up). Either way, you can only change the JavaScript variable from the server code. If that works for you, then fine, but I still think you need to understand the page life cycle very well to make sure that this is what you want.

Avrohom Yisroel Over a year ago

By the way, my last comment about Blazor still stands, as it allows you to do this sort of thing on the fly in ways that WebForms can't do.

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