Retrieve and loop through comments with AJAX and jQuery

I’m working on a custom theme, that is basically a one-page blog, with posts being called inside a slider. The slider navigation works using pagination, but with links displaying like a timeline and showing the date of each post.

I want to display and manipulate the comments for each post outside the slider, so I’m trying to load them using ajax and a jQuery click function.

This is the jQuery click function, that at the moment resides in my header:

$(".timeline-entry a").on('click', function(event) {
      event.preventDefault();
      var myComments = '<?php echo json_encode(get_comments('post_id=72')); ?>'; //for testing with a hard-coded post number...
      $.ajax({
         url: '<?php echo admin_url('admin-ajax.php'); ?>',
	type: 'POST',
	action: 'do_ajax',
         data: {
            'post_id' : '72'
         },
         dataType: 'json',
         success: function(myResult) {
	   //$.each (myResult, function() {
               //alert(myResult.comment_author);
	   //}); Can't seem to retrieve any comments or data at all
	   alert("sOmethingishappenin'"); //This is alerted, so the AJAX is working?!
         },
         error: function(error) {
            alert(error);
         }
      });
   });

This is in my functions.php:

add_action('wp_ajax_nopriv_do_ajax', 'retrieve_comments');
add_action('wp_ajax_do_ajax', 'retrieve_comments');

function retrieve_comments(){
   $myPost = $_REQUEST['post_id'];
   $output = get_comments('post_id=' + $myPost);
   $output = json_encode($output);
   if(is_array($output)){
      print_r($output);
   }
   else{
      echo $output;
   }
   die;
};

I put this line in for testing var myComments = '<?php echo json_encode(get_comments('post_id=72')); ?>'; and the array that is returned looks like this:

[{"comment_ID":"2","comment_post_ID":"72","comment_author":"Mr WordPress","comment_author_email":"","comment_author_url":"http:\/\/wordpress.org\/","comment_author_IP":"","comment_date":"2010-07-11 12:10:08","comment_date_gmt":"2010-07-11 12:10:08","comment_content":"Hi, this is a comment.<br \/>To delete a comment, just log in and view the post's comments. There you will have the option to edit or delete them.","comment_karma":"0","comment_approved":"1","comment_agent":"","comment_type":"","comment_parent":"0","user_id":"0"}

But using the AJAX function, I can’t seem to grab any of this information. So far, I can only get a string to show as an alert. If I try anything related to myResult I only get 0.

Can you help?!…