Calling C# code behind method using javascript without ajax

Question

I have 2 buttons in my asp.net File

<asp:Button ID="BTN_Send_LA" runat="server" Text="Save" OnClientClick="ConfirmSendData()"></asp:Button>
//The button the client will click

<asp:Button ID="UploadButton" runat="server" Text="" OnClick="BTN_Send_LA_Click"/>
//Dummy Button for the JS .click()

And here is my Js part:

function ConfirmSendData() {
    var r = confirm("Êtes vous bien: " + document.getElementById("<%=DDL_LaveurLA.ClientID%>").options[document.getElementById("<%=DDL_LaveurLA.ClientID%>").selectedIndex].text + " sinon veuillez changer dans le champ spécifié 'Laveur'");

    if (r == true) {

        var clickButton = document.getElementById("<%= UploadButton.ClientID %>");
        clickButton.click();

        //$('UploadButton').trigger('click'); TEST 1
        //__doPostBack not working aswell

    }
}

So here what i expect to be done:

1. The client click the first button (Trigger the JS) => Works
2. R is true => Works
3. The JS part trigger the Onclick of UploadButton => Don't Work

I don't understand why this method doesn't work as it seems to be the general approach most other answers take on StackOverflow?

UPDATE:

Ok, I've tried every solutions proposed below and now i have weird problems:

When I click on the client button, 1 of the 3 following things happens randomly (route followed with the debugger)

1: The button click do a blank postback (IsPostBack == true) event OnClick="BTN_Send_LA_Click" not fired

2: The button click do a blank postback (IsPostBack == false) event OnClick="BTN_Send_LA_Click" not fired

3: The button fire the event OnClick="BTN_Send_LA_Click" of the dummy button properly.

I don't understand why. When i click directly on the dummy button, everything works fine

Everytime I do a CTRL+F5, the first time I click the client button will work 100% (event fired)

something else: in my event BTN_Send_LA_Click(), I change the background color of multiple controls (lightgreen)

1: If I click on the dummy button => the background color of the controls are changed

2: If I click on the client button and even if the BTN_Send_LA_Click() is fired, the background color doesn't change.

Why ? I'm totally lost on this one

Updated code:

        function ConfirmSendData()
     {
            /*
            var dd = document.getElementById("<%=DDL_LaveurLA.ClientID%>");
            var txt = dd.options[dd.selectedIndex].text;
            var r = confirm("Êtes vous bien: " + txt + " sinon veuillez changer dans le champ spécifié 'Laveur'"); */

            var r = confirm("Êtes vous bien: " + document.getElementById("<%=DDL_LaveurLA.ClientID%>").options[document.getElementById("<%=DDL_LaveurLA.ClientID%>").selectedIndex].text + " sinon veuillez changer dans le champ spécifié 'Laveur'");

            if (r == true) {
                //$("#<%=UploadButton.ClientID%>").click();

                var clickButton = document.getElementById("<%= UploadButton.ClientID %>");
                clickButton.click();

            }
            return false;  
    }

Answer 1

You've got it all right except:

1. You need a closing } on your if statement.
2. ConfirmSendData() needs to return false to prevent the first button from submitting.

i.e.

function ConfirmSendData() {
    var r = confirm("Êtes vous bien...");
    if (r == true) {
        var clickButton = document.getElementById("<%= UploadButton.ClientID %>");
        clickButton.click();
    }
    return false;
}