title: How to redirect to prefixed path in Expressjs Router
body: <p>I am deploying an app in a specific path say under <code>/path</code>. </p>
  
  <pre><code>// login form
  router.get('/login', function (req, res) {
  })
  router.post('/login', function(req, res) {
    if (success) {
       res.redirect('/') // &lt;&lt;--- does not direct to /path
    }
  })
  // install router under path
  app.use('/path', router)
  </code></pre>
  
  <p>So I can navigate to <code>http://.../path/login</code>
  and after logged in, I want to redirect the user to the <code>/</code> under <code>/path</code>.</p>
  
  <p>I don't want to hardcode <code>/path</code> and only use relative path since router has already been mounted under <code>path</code>.</p>
  
  <p>Is it possible? How to do that?</p>