title: How to throw a 404 error in express.js?
body: <p>In app.js, I have</p>
  
  <pre><code>// catch 404 and forward to error handler
  app.use(function(req, res, next) {
    var err = new Error('Not Found');
    err.status = 404;
    next(err);
  });
  </code></pre>
  
  <p>so if I request some not exist url like <code>http://localhost/notfound</code>, above code will execute.</p>
  
  <p>In exist url like <code>http://localhost/posts/:postId</code>, I would like to throw 404 error when access some not exist postId or deleted postId.</p>
  
  <pre><code>Posts.findOne({_id: req.params.id, deleted: false}).exec()
    .then(function(post) {
      if(!post) {
        // How to throw a 404 error, so code can jump to above 404 catch?
      }
  </code></pre>