[Rails] Figured out. Thank you all for the posts

Thanks to Colin Law, Timothy G., Stephan Wehner, Greg Akins, Gautam Pai.
Here is what I got.

1. Ajax is a bad idea for this. It will waste bandwidth and more
importantly makes the character counting unreliable. You do not know
when the response will be.

2. Javascript is supposed to be the tool to handle this type of tasks.
Both jquery and prototype does the work. I did not know which event to
respond to and how to attach event handlers to the element. Now I know.

a) Default place for the javascript code. "application.js". No other
changes is needed. However, if you would like to load code like this
dynamically into your application.js, you are welcome to do so.

b). For jquery, the code could be the following:
====================================================================
$(document).ready(function () {
$('#id_of_text_area').keyup(function(){
var s = 100 - $(this).val().length;
$('#id_of_message').text("You have " + s + " characters left");
});
});
====================================================================

The key thing which frustrated me was the $(document).ready. That makes
sure the code is executed at the time when the page is ready, thus
establishing keyup event handler for .keyup event of the element of
concern, i.e. the one whose input needs to be counted. The same is true
for any event handler if it needs to be established by the time the page
is ready.

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