title: How to check session in Node.js Express?
body: <p>I try to check if session in Express 4 is exist:</p>
  
  <pre><code>if(req.session.user == undefined) {}
  </code></pre>
  
  <p>It gives me error:</p>
  
  <pre><code> Cannot read property 'user' of undefined
  </code></pre>
  
  <p>How I can check if exist value in session?</p>