title: JavaScript - How does Disqus handle its "disqus_config" function which contains undefined property?
body: <p>Here is a part of the Disqus's "universal code":</p>
  
  <pre><code>var disqus_config = function () {
      this.page.url = PAGE_URL;  // Replace PAGE_URL with your page's canonical URL variable
      this.page.identifier = PAGE_IDENTIFIER; // Replace PAGE_IDENTIFIER with your page's unique identifier variable
  };
  </code></pre>
  
  <p>What I don't undestand is how Disqus handle this function, because <code>page</code> is <code>undefined</code>, so we cannot access to <code>identifier</code> or <code>url</code>.
  I have tested several examples:</p>
  
  <pre><code>disqus_config();
  console.log(disqus_config.page);
  var a = new disqus_config();
  </code></pre>
  
  <p>But I still don't understand how Disqus handle this <code>undefined</code> element.</p>