Boilerplate code for making AJAX calls to wordpress.

ajax_class.php

<?php
if (!defined('ABSPATH')) {
    exit;
}

class ajax_class
{
    public function __construct()
    {
      //Hook = 'wp_ajax_{$action}' where $action must equal the AJAX request's 'action' property.  
      add_action('wp_ajax_do_something', array($this,'function_do_something'));
      //Hook = 'wp_ajax_nopriv_{$action}'. This action is triggered if the client making the request is not logged in to wordpress
      add_action('wp_ajax_nopriv_do_something', array($this,'function_must_login'));
    }

    public function function_do_something() {

       // Grab request data
       $foo = $_REQUEST["foo"];

       //Set results to be sent back to javascript
       if(!empty($foo)){
         $result['msg'] = $foo;
       }
       else{
         $result['msg'] = 'ERROR';
       }

       //Send result back to ajax request
       if(!empty($_SERVER['HTTP_X_REQUESTED_WITH']) && strtolower($_SERVER['HTTP_X_REQUESTED_WITH']) == 'xmlhttprequest') {
          $result = json_encode($result);
          echo $result;
       }
       else {
          header("Location: ".$_SERVER["HTTP_REFERER"]);
       }

       //if die() function is not called ajax request will hang.
       die();

    }

    public function function_must_login() {
       _e( 'You Are Not Logged In', 'textdomain' );
       die();
    }

}

new ajax_class();
?>

index.php

<?php
  include_once('ajax_class.php');

  // Register style sheet.
  add_action( 'admin_enqueue_scripts', 'source_javacript' );

  function source_javacript() {

  	wp_enqueue_script('jquery');
  	wp_enqueue_script('my_ajax',plugins_url('my_ajax.js', __FILE__), array('jquery'), null, true);
  	wp_localize_script('my_ajax', 'js_data', array('ajaxurl' => admin_url( 'admin-ajax.php' )) );

  }
?>

my_ajax.js

jQuery(document).ready(function() {

  jQuery.ajax({
     type : "get",
     dataType : "json",
     url : js_data.ajaxurl, //URL to your wordpress install's admin-ajax.php file
     data : {action: "do_something", foo : "bar" },
     success: function(response) {
        alert(response.msg);
     }
  });
  
});