title: How to make javascript work as a response to php events?
body: <p>I am developing a website for practice, and I would like to know how to use JS to notify the user that the username he picked is already in use, all works fine, if my function(check_username) returns false, the user succesfully registers himself into the site, otherwise the register won't happen. </p>
  
  <p>When the user can't register I would like to know how can I notify the user with a js script.   </p>
  
  <pre><code>&lt;?php 
      //database includes
      include_once('../config/init.php');
      include_once('../database/users.php');
  
      if(!check_username($_POST['username'])) {
          insertUser($_POST['name'], $_POST['username'], $_POST['email'], $_POST['pass']);
      }
      else header('Location: ../index.php');
  ?&gt;
  </code></pre>