How to query post content with javascript?

Hi All,
This seems like it should be easy, but I’ve been pulling my (thinning) hair out getting it to work. What it boils down to is, how do I get the content of a post from the wordpress database and replace the contents of an HTML <div> with that content?

At page load time, I have php that generates the post ID for the corresponding image, i.e.:
onmousedown="javascript:getDescription('<?php the_ID(); ?>');"

That onmousedown() calls a javascript that passes the value of the post I want to query to a php file, i.e.:

<script type="text/javascript">
                function getDescription(for_id) {
                    $.ajax({
                        type: 'GET',
                        url: "<?php bloginfo('template_url'); ?>/assets/includes/get-description.php?id=" + for_id,
                        success: function(data, textStatus, jqXHR){
                            $('#textdescription').html(data);
                             }
                        });
                }
        </script>

The php file get-description.php then should do the query and return the post contents, which the javascript then uses to update the <div> contents:

<p><?php
$contentVar = $_GET['id'];
$post = get_post($contentVar);
$content = $post->post_content;
echo $title;
?></p>

So this is all tested and working, except for the php file that is supposed to do the database query— I can’t get it to return anything when I query. I can return other content including my incoming post id, so I know all the pipes are connected and working.

Any help appreciated, thanks in advance!