AJAX可以与Sinatra一起使用吗？

This has been asked a few times, however, there seems to be no good answer on the web. To avoid replies or links which do not answer this question, I will rephrase it. A page has only a form with input text (and button). We post text to the server and want it just sent back and alerted. In other words, the view should have the line:

$.post(...,$("form").serialize(),function(reply){alert(reply);}); 

With PhP, the answer is an "echo" one-liner. Can this be done in Sinatra - I understand that backend frameworks are built around handling DOM manipulation themselves. I am asking this question because it would be nice to use a more expressive language such as Ruby also just for db interface/backend logic.

Edit: This is the /views/index.erb (with "/reply" in place of "reply.php" - otherwise index.html/index.php) part:

<script src="jquery"></script>

<form action="reply.php" method="post"><input type="text" name="t"></form> 

<script>
$(document).ready(function(){
   $("form").submit(function(event){
      event.preventDefault();
      $.post("reply.php",$("form").serialize(),function(reply){alert(reply);});
   });
});
</script>

Note that with event.preventDefault() we stay at / route and don't go into /reply.php, which would remove the form and print just the submitted text. In other words, this is what enables Ajax and we get a reply (only in alert) from the server with reply.php:

echo $_POST["t"];

With Sinatra, we need to have a routes.rb controller:

require 'sinatra'

get '/' do
  erb :index
end

post '/reply' do
  ...
end

Question: With index.html/reply.php we get an alert with posted text. How can index.erb/routes.rb be modified also to get an alert with posted text?

Yes, this can be done with Sinatra. Any route can return data from the server to the client; by default this is sent with a text/html mime type, but you can also return plain text with an appropriate mime type:

post "/reply.php" do
  content_type :text
  params['t']
end

If you want to echo more than just one value back, you can see all the request values by using the code from this question.

This will send the value of your variable back from the server, and the client-side JavaScript code that you have in your question will cause an alert() to show up.