Connect Middleware

I absolutely love Connect, the framework that powers Express. Much like Rack it is all about middleware, leading to wonderfully modular organization of code.

What Is Middleware?

Middleware functions are used by an application, in order, for each request / response pair. Middleware is used for parsing POST body data, CSRF protection, authenticating the user from the session, and all the other important pre-route work. This is what it looks like:

function middleware(req, res, next) {
   // call the next middleware (completed successfully)
   next()

   // skip to error handling middleware
   next(new Error)
}

app.use(middleware)

Higher Order Middleware

One useful thing you can do is write higher-order helper functions that return middleware. A common use case is finding the model based on the :id in a route:

// api/middleware.js

function find(Model) {
   function middleware(req, res, next) {
      Model.find(req.params.id, function(error, model) {
         if (error) { next(error); }
         else { req.model = model; next(); }
      }
   }
   return middleware;
}

module.exports = {
  find: find
};

Middleware for a route is included as an extra param before the req/res callback.

// api/users.js

var express, helpers, User, app;

express = require("express");
middleware = require("./middleware");
User = require("../models/user");

app = express();

app.get("/users/:id", middleware.find(User), function(req, res) {
   res.render(req.model.toJSON());
});

module.exports = app;

Modularity

Apps can be added as middleware to other apps, making express apps recursively modular. For example, the users api app can be used in the main application.

// app.js

var express, app, api = {};

express = require("express");
api.users = require("./api/users");

app = express();

app.get("/", function(req, res) {
   res.render("index");
});

app.use(api.users);

app.listen(process.env.PORT);

Appending Routes

App middleware pieces can also be used at a specific route. For instance you could create an app module for authentication, and mount it:

auth = express();
auth.get("/twitter", fn);
auth.get("/twitter/callback", fn);

// export and require auth

app.use("/auth", auth);