An Express middleware to check the requested content type.

acceptedContentType.js

/**
 * A middleware to check the requested content type.
 * Note: 
 * Just be sure to place the middleware before:
 * app.use(app.router);
 */
module.exports.acceptedContentType = function () {

  return function (req, res, next) {

    res.format({
      'application/xml': function () {
        next();
      },
      'application/json': function () {
        next();
      },
      'default': function () {
        res.send(406, 'Sorry, we only provide resources in JSON and XML.');
      }
    });
  };
};

You might want to read a great article about how to write middleware for Express:
http://www.hacksparrow.com/how-to-write-midddleware-for-connect-express-js.html