This is short story of my understanding of express.js web application stack. All source code below is in CoffeeScript, which I find more readable and easier to write than vanilla JavaScript.

Minimal express.js app

Express.js is actually very thin (yet powerful) layer over node’s http stuff. Let’s start with minimal example:

express = require 'express'

app = express()

app.configure ->
    app.use express.bodyParser()
    app.use express.cookieParser()
    app.use app.router
    app.use express.static "#{__dirname}/public"

app.get '/', (req, res) ->
    res.send '<body>Hello</body>'

app.listen 3000

Here’s what happens when GET request comes:

  1. bodyParser middleware is called - it decodes different kinds of http request body.
  2. cookieParser is called - it decodes Cookie headers and populates req.cookies.
  3. app.router is called. It traverses app.routes array to find matching route using regexps. If it finds one, route is being executed and all ends here. If it doesn’t express proceeds to next middleware.
  4. Next middleware - static is called. It checks for matching static files in /public directory. When found - file is being sent back to client in response.

Routes internal object

Here’s app.routes content for above example (via console.log app.routes):

{ get:
    [ { path: '/',
        method: 'get',
        callbacks: [Object],
        keys: [],
        regexp: /^\/\/?$/i } ] }

app.routes object was automatically populated by app.get call.

Custom application middleware

Every middleware takes three arguments:

customMiddleware = (req, res, next) ->
    console.log "Hello from middleware! User IP: #{req.ip}"

req and res are request and response objects and next is a callback which is set by app.use to next middleware in request processing chain. Try putting following line after cookieParser:

app.use customMiddleware

Running application and requesting will yield following line on the console for every request:

Hello from middleware! User IP:

It is important to call next inside middleware. In other case request processing chain will stop on your middleware (what can be desired in some cases).