Calling Code-behind from Javascript

Question

On the click of a button, I call a JavaScript function. After getting the value, I need to perform some stuff from the value obtained in the code-behind. How should I call code-behind?

My aspx:

function openWindow(page) {
  var getval = window.showModalDialog(page);
  document.getElementById("<%= TxtInput.ClientID %>").value = getval; 
  //After this I need to perform stuff 'Upload(TxtInput.value)' into database from the code-behind
}

The button calling the function is set up in the following manner:

<button class="doActionButton" id="btnSelectImage" runat="server" onclick="openWindow('../rcwksheet/popups/uploader.htm')">Select Image</button>

My desired code behind (VB):

Public Sub btnSaveImage_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnSelectImage.ServerClick
  Dim inputFile As String = Me.TxtInput.Value
  //do more stuff here
End Sub

So:

1. Is there a way to call code-behind from the JavaScript?
2. Can I somehow use the "onclick" property of a button to first go to a JavaScript and then to the code-behind?
3. Trigger a code-behind call "onchange" of the TxtInput.Value?

Answer 1

yes there is a way.

first, you can use javascript to submit the form after your return value is set in TxtInput.

function openWindow(page) {
  var getval = window.showModalDialog(page);
  document.getElementById("<%= TxtInput.ClientID %>").value = getval; 
  document.forms[0].submit();
}

then in your code behind, you can handle TxtInput's value in page load event.

protected void Page_Load(object sender, EventArgs e)
{
    if (Page.IsPostBack)
    {
        if (this.Input.Value != string.Empty)
        {
            this.Input.Value += "blah";
        }
    }
}

note: you may need Identifying control that caused postback