ExpressJS

When it comes to build web applications using Node.js, creating a server can take a lot of time. Over the years Node.js has matured enough due to the support from community. Using Node.js as a backend for web applications and websites help the developers to start working on their application or product quickly. In this tutorial, we are going to look into Expressjs which is a Node.js framework for web development that comes with features like routing and rendering and support for REST APIs.

What is Express?

Express is the most popular Node.js framework because it requires minimum setup to start an application or an API and is fast, and unopinionated at the same time. In other words, it does not enforces its own philosophy that a application or API should be built in a specific way, unlike Rails and Django. Its flexibility can be calculated by the number of npm modules available which makes it pluggable at the same time. If you have basic knowledge of HTML, CSS, and JavaScript and how Node.js works in general, in no time you will be able to get started with Expressjs.

Express was developed by TJ Holowaychuk and is now maintained by Node.js foundation and open source developers. To get started with the development using Express, you need to have Node.js and npm installed. You can install Node.js on your local machine and along with it comes the command line utility npm that will help us to install plugins or as called dependencies later on in our project.

To check if everything is installed correctly, please open your terminal and type:

node --version
v5.0.0
npm --version
3.5.2

If you are getting the version number instead of an error that means you have installed Node.js and npm successfully.

Why use Expressjs?

Before we start with mechanism of using Express as the backend framework, let us first explore why we should consider it using or the reasons of its popularity.

  • Express lets you build single page, multi-page, and hybrid web and mobile applications. Other common backend use is to provide an API for a client (whether web or mobile).
  • It comes with a default template engine, Jade which helps to facilitate the flow of data into a website structure and does support other template engines.
  • It supports MVC (Model-View-Controller), a very common architecture to design web applications.
  • It is cross-platform and is not limited to any particular operating system.
  • It leverages upon Node.js single threaded and asynchronous model.

Whenever we create a project using npm, our project must have a package.json file.

Creating package.json

A JSON (JavaScript Object Notation) file is contains every information about any Express project. The number of modules installed, the name of the project, the version, and other meta information. To add Expressjs as a module in our project, first we need to create a project directory and then create a package.json file.

mkdir express-app-example
cd express-app-example
npm init --yes

This will generate a package.json file in the root of the project directory. To install any module from npm we need to have package.json file exist in that directory.

{
  "name": "express-web-app",
  "version": "0.1.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "keywords": [],
  "license": "MIT"
}

Installing Express

Now we have package.json file, we can install Express by running the command:

npm install --save express

We can confirm that Express has correctly installed by two ways. First, there will be new section in package.json file named dependencies under which our Express exists:

{
  "name": "express-web-app",
  "version": "0.1.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "keywords": [],
  "license": "MIT",
  "dependencies": {
    "express": "4.16.0"
  }
}

Second way is that a new folder called node_modules suddenly appeared in the root of our project directory. This folder stores the packages we install locally in our project.

Building a Server with Express

To use our installed package for Express framework and create a simple server application, we will create the file, index.js, at the root of our project’s directory.

const express = require('express');
const app = express();

app.get('/', (req, res) => res.send('Hello World!'));

app.listen(3000, () => console.log('Example app listening on port 3000!'));

To start the server, go to your terminal and type:

node index.js

This will start the server. This bare-minimum application will listen on port 3000. We make a request through our browser on http://localhost:3000 and our server will respond with Hello World to which the browser is the client and the message will be shown there.

The first line of our code is using the require function to include the express module. This is how we include and use a package installed from npm in any JavaScript file in our project. Before we start using Express, we need to define an instance of it which handles the request and response from the server to the client. In our case, it is the variable app.

app.get() is a function that tells the server what to do when a get request at the given route is called. It has a callback function (req, res) that listen to the incoming request req object and respond accordingly using res response object. Both req and res are made available to us by the Express framework.

The req object represents the HTTP request and has properties for the request query string, parameters, body, and HTTP headers. The res object represents the HTTP response that an Express app sends when it gets an HTTP request. In our case, we are sending a text Hello World whenever a request is made to the route /.

Lastly, app.listen() is the function that starts a port and host, in our case the localhost for the connections to listen to incoming requests from a client. We can define the port number such as 3000.

Anatomy of an Express Application

A typical structure of an Express server file will most likely contain the following parts:

Dependencies

Importing the dependencies such as the express itself. These dependencies are installed using npm like we did in the previous example.

Instantiations

These are the statements to create an object. To use express, we have to instantiate the app variable from it.

Configurations

These statements are the custom application based settings that are defined after the instantiations or defined in a separate file (more on this when discuss the project structure) and required in our main server file.

Middleware

These functions determine the flow of request-response cycle. They are executred after every incoming request. We can also define custom middleware functions. We have section on them below.

Routes

They are the endpoints defined in our server that helps to perform operations for a particular client request.

Bootstrapping Server

The last that gets executed in an Express server is the app.listen() function which starts our server.

We will now start disussing sections that we haven’t previously discussed about.

Routing

Routing refers to how an server side application responds to a client request to a particular endpoint. This endpoint consists of a URI (a path such as / or /books) and an HTTP method such as GET, POST, PUT, DELETE, etc.

Routes can be either good old web pages or REST API endpoints. In both cases the syntax is similar syntax for a route can be defined as:

app.METHOD(PATH, HANDLER);

Routers are helpful in separating concerns such as different endpoints and keep relevant portions of the source code together. They help in building maintainable code. All routes are defined before the function call of app.listen(). In a typical Express application, app.listen() will be last function to execute.

Routing Methods

HTTP is a standard protocol for a client and a server to communicate over. It provides different methods for a client to make request. Each route has at least on hanlder function or a callback. This callback function determines what will be the response from server for that particular route. For example, a route of app.get() is used to handle GET requests and in return send simple message as a response.

// GET method route
app.get('/', (req, res) => res.send('Hello World!'));

Routing Paths

A routing path is a combination of a request method to define the endpoints at which requests can be made by a client. Route paths can be strings, string patterns, or regular expressions.

Let us define two more endpoints in our server based application.

app.get('/home', (req, res) => {
  res.send('Home Page');
});
app.get('/about', (req, res) => {
  res.send('About');
});

Consider the above code as a bare minimum website which has two endpoints, /home and /about. If a client makes a request for home page, it will only response with Home Page and on /about it will send the response: About Page. We are using the res.send function to send the string back to the client if any one of the two routes defined is selected.

Routing Parameters

Route parameters are named URL segments that are used to capture the values specified at their position in the URL. req.params object is used in this case because it has access to all the parameters passed in the url.

app.get('/books/:bookId', (req, res) => {
  res.send(req.params);
});

The request URL from client in above source code will be http://localhost:3000/books/23. The name of route parameters must be made up of characters ([A-Za-z0-9_]). A very general use case of a routing parameter in our application is to have 404 route.

// For invalid routes
app.get('*', (req, res) => {
  res.send('404! This is an invalid URL.');
});

If we now start the server from command line using node index.js and try visiting the URL: http://localhost:3000/abcd. In response, we will get the 404 message.

Middleware Functions

Middleware functions are those functions that have access to the request object (req), the response object (res), and the next function in the application’s request-response cycle. The objective of these functions is to modify request and response objects for tasks like parsing request bodies, adding response headers, make other changes to request-response cycle, end the request-response cycle and call the next middleware function.

The next function is a function in the Express router which is used to execute the other middleware functions succeeding the current middleware. If a middleware function does include next() that means the request-response cycle is ended there. The name of the function next() here is totally arbitary and you can name it whatever you like but is important to stick to best practices and try to follow a few conventions, especially if you are working with other developers.

Also, when writing a custom middleware do not forget to add next() function to it. If you do not mention next() the request-response cycle will hang in middle of nowhere and you servr might cause the client to time out.

Let use create a custom middleware function to grasp the understanding of this concept. Take this code for example:

const express = require('express');
const app = express();

// Simple request time logger
app.use((req, res, next) => {
   console.log("A new request received at " + Date.now());

   // This function call tells that more processing is
   // required for the current request and is in the next middleware
   function/route handler.
   next();  
});

app.get('/home', (req, res) => {
  res.send('Home Page');
});

app.get('/about', (req, res) => {
  res.send('About Page');
});

app.listen(3000, () => console.log('Example app listening on port 3000!'));

To setup any middleware, whether a custom or available as an npm module, we use app.use() function. It as one optional parameter path and one mandatory parameter callback. In our case, we are not using the optional paramaeter path.

app.use((req, res, next) => {
  console.log('A new request received at ' + Date.now());
  next();
});

The above middleware function is called for every request made by the client. When running the server you will notice, for the every browser request on the endpoint /, you will be prompt with a message in your terminal:

A new request received at 1467267512545

Middleware functions can be used for a specific route. See the example below:

const express = require('express');
const app = express();

//Simple request time logger for a specific route
app.use('/home', (req, res, next) => {
  console.log('A new request received at ' + Date.now());
  next();
});

app.get('/home', (req, res) => {
  res.send('Home Page');
});

app.get('/about', (req, res) => {
  res.send('About Page');
});

app.listen(3000, () => console.log('Example app listening on port 3000!'));

This time, you will only see a similar prompt when the client request the endpoint /home since the route is mentioned in app.use(). Nothing will be shown in the terminal when the client requests endpoint /about.

Order of middleware functions is important since they define when to call which middleware function. In our above example, if we define the route app.get('/home')... before the middleware app.use('/home')..., the middleware function will not be invoked.

Third Party Middleware Functions

Middleware functions are useful pattern that allows developers to reuse code within their applications and even share it with others in the form of NPM modules. The essential definition of middleware is a function with three arguments: request (or req), response (res), and next which we observer in the previous section.

Often in our Express based server application, we will be using third party middleware functions. These functions are provided by Express itself. They are like plugins that can be installed using npm and this is why Express is flexible.

Some of the most commonly used middleware functions in an Express appication are:

bodyParser

It allows developers to process incoming data, such as body payload. The payload is just the data we are receiving from the client to be processed on. Most useful with POST methods. It is installed using:

npm install --save body-parser

Usage:

const bodyParser = require('body-parser');

// To parse URL encoded data
app.use(bodyParser.urlencoded({ extended: false }));

// To parse json data
app.use(bodyParser.json());

It is probably one of the most used third-party middleware function in any Express applicaiton.

cookieParser

It parses Cookie header and populate req.cookies with an object keyed by cookie names. To install it,

$ npm install --save cookie-parser
const cookieParser = require('cookie-parser');
app.use(cookieParser());

session

This middleware function creates a session middleware with given options. A session is often used in applications such as login/signup.

$ npm install --save session
app.use(
  session({
    secret: 'arbitary-string',
    resave: false,
    saveUninitialized: true,
    cookie: { secure: true }
  })
);

morgan

The morgan middleware keeps track of all the requests and other important information depending on the output format specified.

npm install --save morgan
const logger = require('morgan');
// ... Configurations
app.use(logger('common'));

common is a predfined format case which you can use in the application. There are other predefined formats such as tiny and dev, but you can define you own custom format too using the string parameters that are available to us by morgan.

A list of most used middleware functions is available at this link.

Serving Static Files

To serve static files such as CSS stylesheets, images, etc. Express provides a built in middleware function express.static. Static files are those files that a client downloads from a server.

It is the only middleware function that comes with Express framework and we can use it directly in our application. All other middlewares are third party.

By default, Express does not allow to serve static files. We have to use this middleware function. A common practice in the development of a web application is to store all static files under the ‘public’ directory in the root of a project. We can serve this folder to serve static files include by writing in our index.js file:

app.use(express.static('public'));

Now, the static files in our public directory will be loaded.

http://localhost:3000/css/style.css
http://localhost:3000/images/logo.png
http://localhost:3000/images/bg.png
http://localhost:3000/index.html

Multiple Static Directories

To use multiple static assets directories, call the express.static middleware function multiple times:

app.use(express.static('public'));
app.use(express.static('files'));

Virtual Path Prefix

A fix path prefix can also be provided as the first argument to the express.static middleware function. This is known as a Virtual Path Prefix since the actual path does not exist in project.

app.use('/static', express.static('public'));

If we now try to load the files:

http://localhost:3000/static/css/style.css
http://localhost:3000/static/images/logo.png
http://localhost:3000/static/images/bg.png
http://localhost:3000/static/index.html

This technique comes in handy when providing multiple directories to serve static files. The prefixes are used to help distinguish between the multiple directories.

Template Engines

Template engines are libraries that allow us to use different template languages. A template language is a special set of instructions (syntax and control structures) that instructs the engine how to process data. Using a template engine is easy with Express. The popular template engines such as Pug, EJS, Swig, and Handlebars are compatible with Express. However, Express comes with a default template engine, Jade, which is the first released version of Pug.

To demonstrate how to use a Template Engine, we will be using Pug. It is a powerful template engine that provide features such as filters, includes, interpolation, etc. To use it, we have to first install as a module in our project using npm.

npm install --save pug

This command will install the pug and to verify that installed correctly, just take a look at the package.json file. To use it with our application first we have to set it as the template engine and create a new directory ‘./views’ where we will store all the files related to our template engine.

app.set('view engine', 'pug');
app.set('views', './views');

Since we are using app.set() which indicates configuration within our server file, we must place them before we define any route or a middleware function.

In the views direcotry, create file called index.pug.

doctype html
  html
    head
      tite="Hello from Pug"
    body
      p.greetings Hello World!  

To run this page, we will add the following route to our application.

app.get('/hello', (req, res) => {
  res.render('index');
});

Since we have already set Pug as our template engine, in res.render we do not have to provide .pug extension. This function renders the code in any .pug file to HTML for the client to display. The browsers can only render HTML files. If you start the server now, and visit the route http://localhost:3000/hello you will see the output Hello World rendered correctly.

In Pug, you must notice that we do not have to write closing tags to elements as we do in HTML. The above code will be rendered into HTML as:

<!DOCTYPE html>
<html>
   <head>
      <title>Hello from Pug</title>
   </head>

   <body>
      <p class = "greetings">Hello World!</p>
   </body>
</html>

The advantage of using a Template Engine over raw HTML files is that they provide support for performing tasks over data. HTML cannot render data directly. Frameworks like Angular and React share this behaviour with template engines.

You can also pass values to template engine directly from the route handler function.

app.get('/', (req, res) => {
  res.render('index', { title: 'Hello from Pug', message: 'Hello World!' });
});

For above case, our index.pug file will be written as:

doctype html
  html
    head
      title= title
    body
      h1= message

The output will be the same as previous case.

Project Structure of an Express App

Since Express does not enforces much on the developer using it, sometimes it can get a bit overwhelming to what project structure one should follow. It does not has a defined structure officially but most common use case that any Node.js based application follows is to separate different tasks in different modules. This means to have separate JavaScript files.

Let us go through a typical strucutre of an Express based web application.

project-root/
   node_modules/          // This is where the packages installed are stored
   config/
      db.js                // Database connection and configuration
      credentials.js       // Passwords/API keys for external services used by your app
      config.js            // Environment variables
   models/                 // For mongoose schemas
      books.js
      things.js
   routes/                 // All routes for different entities in different files
      books.js
      things.js
   views/
      index.pug
      404.pug
        ...
   public/                 // All static files
      images/
      css/
      javascript/
   app.js
   routes.js               // Require all routes in this and then require this file in
   app.js
   package.json

This is pattern is commonly known as MVC, model-view-controller. Simply because our database model, the UI of the application and the controllers (in our case, routes) are written and stored in separate files. This design pattern that makes any web application easy to scale if you want to introduce more routes or static files in the future and the code is maintainable.