title: URL of open(...) relative or absolute
body: <p>I don't quite understand the functioning of the <code>url</code> parameter of XMLHttpRequest <code>open(method, url, async)</code>. Let's say I have a web server like that:</p>
  <p><a href="https://i.stack.imgur.com/CJQoE.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/CJQoE.png" alt="Enter image description here" /></a></p>
  <p><code>page.html</code> sends an asynchronous request to <code>controller.php</code>. As we can only send requests to our own web server, I assume that we don't have to rewrite the website's name in the URL.</p>
  <p><strong>Example:</strong> instead of <code>open('GET', 'http://www.mywebsite/controller.php', true)</code> we can simply write <code>open('GET', 'controller.php', true)</code>).</p>
  <p>I don't get if this URL is relative to <code>page.html</code> or absolute from the root of the server.</p>
  <hr />
  <p><strong>TL;DR:</strong> Do I have to write <code>open('GET', 'controller.php?param=1', true)</code> or <code>open('GET', 'folder/controller.php?param=1', true)</code>?</p>