expressjs.com/en/api/res-format.jade

section
  h3(id='res.format') res.format(object)
  
  p.
    Performs content-negotiation on the request Accept header
    field when present. This method uses <code>req.accepted</code>, an array of
    acceptable types ordered by their quality values, otherwise the
    first callback is invoked. When no match is performed the server
    responds with 406 "Not Acceptable", or invokes the <code>default</code>
    callback.

  p.
    The Content-Type is set for you when a callback is selected,
    however you may alter this within the callback using <code>res.set()</code>
    or <code>res.type()</code> etcetera.

  p.
    The following example would respond with <code>{ "message": "hey" }</code>
    when the Accept header field is set to "application/json" or "*/json",
    however if "*/*" is given then "hey" will be the response.

  +js.
    res.format({
      'text/plain': function(){
        res.send('hey');
      },
    
      'text/html': function(){
        res.send('<p>hey</p>');
      },
    
      'appliation/json': function(){
        res.send({ message: 'hey' });
      }
    });

  p.
    In addition to canonicalized MIME types you may also
    use extnames mapped to these types, providing a slightly
    less verbose implementation:

  +js.
    res.format({
      text: function(){
        res.send('hey');
      },
    
      html: function(){
        res.send('<p>hey</p>');
      },
    
      json: function(){
        res.send({ message: 'hey' });
      }
    });