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notes from March 6, 2017 FSA on Express.js


A Node Library for Request Handling March 6, 2017

There is a Codeschool course on Express.js.

  1. The Internet
  2. Building with Express

1. The Internet !

Clients and Servers

  • Client requests a resource
  • Server responds with resource
  • Client initiates, server responds, not other direction!
  • These are roles – not technical specs or computer types.

Clients/servers are conceptual roles.

Clients always initiate. Think of a banker. The client approaches the banker to request money. The banker doesn't go around asking people if they want money!

Your laptop is a server. It's connected to the internet and can send responses to a client. It can also be a client, depending on what you're doing!

Web servers

Processes, not physical machines.


Protocol: "Rules of engagement or interaction".

An application-level protocol – used at the app level.

A protocol for communicating on the internet. It's huge. Specification, not implementation. Think of a dance or manners like Emily Post's.

Specifies allowable metadata and content, but not details of metadata and content.

Stateless: state is cleared once communication is done, is not persistent. Doesn't remember previous communication!

Check slides for details about who made HTTP and how it was made.


  Client  ===> 'the internet' : TCP/IP ====> server
  request ===>     routing...          ====> response
  response <====================================|

An HTTP Request is just a message with a certain format.

  • HTTP structures the communication.
  • TCP/IP is the address/routing system that routes the message.

Every request gets exactly one (total) response ... even if TCP slices up the message in transmission, you will always get one response.


  • Route => URI/L
  • Headers => Meta-data about the request like 'Host', Accept, Accept-Encoding
  • Payload => Body, data sent by the request


  • Status => Code and Standard message (i.e. 200 OK)
  • Headers => Meta-data associated with the response
  • Body => payload of response

CRUD - Create, Read, Update, Delete

Common Status Codes:

  • 200 – OK
  • 201 – Created
  • 304 – Cached
  • 400 – Bad Request
  • 401 – Unauthorized
  • 404 – Not Found
  • 500 – Server Error

Creating a Server

You can specify scripts in your NPM package.json, under the scripts key!

  var http = require('http');
  var server = http.createServer(); // <== Nice!

  // When a request comes in...
  server.on('request', function (request, response) {

    // you can check out a request object here
    // you can check out a response object here

    response.end('This is what you get back as a response!');

  // This .on() is similar to an event listener

  // Start listening on port 1337. Once you've started, do callback!
  server.listen(1337, function() {

    console.log('Im awake! Listening on port 1337.');


Port: a communication channel on your computer. Only one program can use a port at a time. Port numbers are identifiers to communicate between programs. Your default port is 80 (localhost). 0-1023 are reserved ports

To connect to a server, you need an IP Address and a port.

Don't forget that your JS may be running on the server-side, not in your browser (client)!

2. Building with Express.js

Express is a pipeline. We tell it what to do along the pipeline as requests come in.

Requests are objects, like an event handlers.

It matches on verb and route and allows chaining with router.

Request / Response cycle:

  • A client initiates a request
  • the server listens and receives the request
  • server processes the request
  • server sends a SINGLE RESPONSE back to the client


In Express middleware === the route handling function. The thing that takes request and response.

If you see 'Cant set headers after they're sent, you are probably trying to send more than one response. A request handler should only ever send one response.

Express takes in the request and looks for a matching request. Is this a GET? Is it a POST? What URL is it? The app processes through the possibilities until it finds one that matches.

  var express = require('express');
  var bodyParser = require('body-parser');
  var app = express() // <== returns an express app, similar to a server
  // A server listens on port, but won't handle requests unless you tell it to.

  app.listen(1337, function() {
      // do stuff

  // You must send back a response. If you don't send a response, the client will just wait!
  app.get('/example', function(request, response) {

  // Sending back HTML
  app.get('/example', function(request, response) {
      // You can send ONE AND ONLY ONE response per request. Not zero! Not more than one!
      response.send('<h3 style="background-color:blue"></h3>');

  // Sending back a file
  app.get('/example', function(request, response) {
      response.sendFile(__dirname + '/example.html');

NOTE: ROUTES ARE NOT FILE PATHS despite how this looks!

  //Sending back JSON
  app.get('/example', function(request, response) {
          name: 'Jason',
          favoriteColor: 'javascript'

  // Looks for any file at the request.url
  app.get('/**', function(request, response) {
      response.sendFile(__dirname + '/' + request.url);

Get and Post are both verbs

  app.get('/example', function(request, response) {
      console.log('coming back to you from a GET request');
  });'/example', function(request, response) {
      console.log('coming back to you from a POST request');

The use of Next()

  app.get('/**', function(request, response, next) {
      response.sendFile(__dirname + '/' + request.url);
  app.get('/**', function(request, response) {

  // This will always match because I have no url parameter!
  app.use(function(request, response, next) {
      console.log('I run always');

Static file serving example

  app.use(function(request, response, next) {
      fs.readFile(__dirName + '/' + request.url, function(err, contents) {
          if (err) {} else {

Send an error 404 to the user

  app.use(function(request, response) {

Query String: request.query

Query string will appear as a question mark after the uri.

  app.get('/times-two', function (request, response) {
    response.send(request.query.number * 2);
  // ^ Will show us any number times two
  // For example: /times-two?number=100 shows 200

Parameters: request.params

request.params is an object where the keys are parts of the url specified by : something. Variable portions of our urls!

  // After `:` are our parameters. /times-two/123 will show us 246
  app.get('/times-two:num', function (request, response) {
    response.send(request.params.num * 2);

Payload: request.body

Get requests can't have a body.

Anytime a request comes in parse the body with JSON, then runs next to continue processing requests

  //'/times-two:num', function (request, response) {

    // Express does not parse the body by itself!
    response.send(request.num * 2);


A layer of route handlers (middlewares). More modular approach to route handling.

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