title: why we use double slash instead http
body: <p>I see some web pages that link javascript file to page like this :</p>
  
  <pre><code>&lt;script async="" src="//www.googletagmanager.com/gtm.js"&gt;&lt;/script&gt;
  </code></pre>
  
  <p>Why we use double slash ( // ) instead <code>http://</code> or <code>https://</code> from beginig absolute url ? what's different ?</p>