Execute jQuery, then execute .NET

Question 1

Try to use OnClientClick method, if you return false it will stop the postback.

http://msdn.microsoft.com/en-us/library/system.web.ui.webcontrols.button.onclientclick.aspx

<asp:Button id="btn" runat="server" OnClientClick="return onclick()" text="Click" />

JavaScript:

function onclick()
{
   // do something
   return false; // this will stop the postback
}

Question 2

Praveen's point (below, downvoted) about async or sync is really important here.

The "OnClientClick" answer will only work for synchronous stuff. If you have AJAX or a Promise, the best pattern I have found is:

1). Stop the default behavior of the button using javascript:

OnClientClick="return false;"

2). create a separate Javascript event handler for the button:

$(document).ready(function (e) {
            $("[id$='btn']").click(function(){

3). Call the client-side ASP validators (skip if you don't have any).

if (Page_ClientValidate())
            {

4). To the AJAX completion function, or in the promise.then() pass a function to submit the form triggering the button click event

__doPostBack('<%= btn.ClientID %>', 'OnClick');

Unless you are dealing with Promises or AJAX, that will all seem really convoluted.

Here is a full example page:

<%@ Page Language="C#" AutoEventWireup="true" CodeFile="aaaaa_test.aspx.cs" Inherits="test" %>

<!DOCTYPE html>

<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title></title>
    <script src="https://ajax.googleapis.com/ajax/libs/jquery/2.2.0/jquery.min.js"></script>
    <script  type="text/javascript">

        $(document).ready(function (e) {

            //2). use the click event to capture the canceled submit.
            $("[id$='cmdSubmit']").click(function(){
                //3). Allow the asp client-side page validators to do their thing...
                if (Page_ClientValidate())
                {
                    ReturnsPromise().then(function () {
                        //4). once the Asynchronous stuff is done, re-trigger the postback
                        //    and make sure it gets handled by the proper server-side handler.
                        __doPostBack('<%= cmdSubmit.ClientID %>', 'OnClick');
                    });
                }
            });

            // very simple promise, usually this woud be an AJAXy thing.....
            //  a placeholder for your real promise.
            function ReturnsPromise()
            {
                //this is just a simple way to generate a promise for this example.
                return Promise.resolve($("[id$='hiddenField']").val($("[id$='txtInput']").val()));
            }

        });

    </script>
</head>
<body>
    <form id="form1" runat="server">
    <div>

        <asp:HiddenField ID="hiddenField" runat="server" />
        Input:<asp:TextBox ID="txtInput" runat="server"></asp:TextBox>
        <asp:RequiredFieldValidator ID="valInput" runat="server" ErrorMessage="Add Text" ControlToValidate="txtInput" EnableClientScript="True" Text="Add Text"></asp:RequiredFieldValidator>
        <br />
        <!-- 1). use client script to stop the default button behavior with OnClientClick="return false;" -->
        <asp:Button ID="cmdSubmit" runat="server" OnClick="cmdSubmit_Click" Text="Submit" OnClientClick="return false;" />

    </div>
    </form>
</body>
</html>

Question 3

First thing is your ajax call async or sync ? you will make ajax call using defer and promise concept

2, Create a hidden button, click the button from jquery after ajax call successful. and real button return false, so it will not perform postback.