TypeError : Router.user() requires a middleware function but got a undefined

 This most probably indicates that your api is not being set properly. Check whether the api file ahas module.exported , you properly express.Router and returning it. In my case I forgot to return the api, and got the error. 

module.exports = function(app, express) {

    var api = express.Router();

/////////////////////////////////////////////////////////////

    api.get("/users", function(req, res) {

        User.find({} , function(err, users) {

            if (err)

            {

                res.send(err);

            }

            else 

            {

                res.json(users);

            }

        });

    } );

/////////////////////////////////////////////////////////////

    return api; //<= forgot this!

}