Web App + REST API with Express, PostgreSQL and Nunjucks on Yarn

In this article I'll show you how to create a basic web application in Node from scratch. I'll be using yarn instead of npm. I issue HTTP requests using httpie instead of curl.

Let's start with an empty directory:

cd myapp
$ yarn init

Install express

yarn add express

Inside index.js, add first (root) route which renders a simple text:

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

app.get('/', (req, res) => {
  res.send('Hello World')
})

app.listen(3000, () => {
  console.log('listening on 3000')
})

Auto-reloading

Install nodemon to restart the server when there is a change

yarn add nodemon --dev

--dev as Nodemon is used only for development.

You can run it by hand

./node_modules/.bin/nodemon server.js

or directly with yarn (as it detects binaries)

yarn run nodemon

or you can put it into package.json (for later customization)

"scripts": {
  "dev": "nodemon index.js"
}

and to be run as

yarn run dev

HTML view

Create index.html in the root of the project:

<!DOCTYPE html>
<h1>Hello world from a HTML file</h1>

Add path dependency at the top of index.js:

const path = require('path')

Add new route, let’s call it /html

app.get('/html', (req, res) => {
  res.sendFile(path.join(__dirname, 'index.html'))
})

Open localhost:3000/html in your browser to check if it works.

Request logger

I’ll use morgan as request logger. Let’s install it

yarn add morgan

morgan is a middleware i.e. a plugin which changes the request or response object before they get handled by the application itself.

Connect the request logger middleware with the express server

const logger = require('morgan');

app.use(logger('dev’));

Static assets

Let’s specify that public/ directory should be used to serve static assets (stylesheets, javascript files, images, etc.). This is an example of a built-in middleware.

app.use(express.static(__dirname + '/public'));

Create the public/ folder in your project’s root directory

mkdir public

Add a simple CSS file. Let’s name it styles.css

body {
  color: red
}

Reference the file from your HTML file:

<link rel="stylesheet" href="styles.css" media="screen">

Open localhost:3000/html in your browser to check if it works.

Form and parsing request body

How to handle form submission in Express ? Let’s start by adding a simple form to index.html.

<form action="/calculate" method="POST">
  <input type="text" name="name" placeholder="name">
  <input type="text" name="amount" placeholder="amount">
  <button type="submit">Submit</button>
</form>

Let’s add a POST route (that matches action attribute from the form) to handle the form submission.

app.post('/calculate', (req, res) => {
  const { name, amount } = req.body || {};
  res.send(`I've got a POST with request body: ${name}, ${amount}`)
})

Express doesn’t handle reading data from the request’s body. We have to use another middleware, called body-parser, to explicitly parse it. Let’s install the middleware

yarn add body-parser

and add it to our Express application.

const bodyParser = require('body-parser')
...
app.use(bodyParser.urlencoded({ extended: true }));
app.use(bodyParser.json());

urlencoded() extracts the data submitted via the form (i.e. the content type is application/x-www-form-urlencoded) and adds them as body property to the request object. the same way json() extracts the data submitted as JSON (i.e. the content type is application/json) and also adds them to the request object.

app.post('/calculate', (req, res) => {
  const { name, amount } = req.body || {};

  res.send(`I've got a POST with request body: ${name}, ${amount}`)
})

Submit the form again to check if you see its values.

Templates with Nunjucks

Plain HTML has its limits. Express supports a variety of template engines. I prefer Nunjucks as it’s just HTML with additional tags. Let’s install nunjucks:

yarn add nunjucks

Register it as Express template engine

const nunjucks = require('nunjucks')

nunjucks.configure('views', {
  autoescape: true,
  express   : app
});

Create views/ directory and move our index.html inside. Adjust h1 slightly as shown below:

<h1>Hello {{ name }} from a HTML file</h1>

Replace the root route with the following:

app.get('/', (req, res) => {
  res.render('index.html', { name: 'Zaiste'} )
})

Open localhost:3000/ in your browser to check if it works.

PostgreSQL

Let’s start by installing pg-promise. It is a library which abstracts away low-level connection management and allows to focus on the business logic. Additionally, it includes a powerful query formatting engine along with the support for automated transactions.

yarn add pg-promise

Create the database

createdb widgets

Create a SQL script that describes widgets table and enters some data into it:

DROP DATABASE IF EXISTS widgets;
CREATE DATABASE widgets;

\c widgets;

CREATE TABLE widgets (
  ID SERIAL PRIMARY KEY,
  name VARCHAR,
  amount INTEGER
);

INSERT INTO widgets (name, amount)
  VALUES ('Widget 1', 33), ('Widget 2', 55), ('Widget 3', 88);

Run the script:

$ psql -f widgets.sql

Add CRUD routes

const widgets = require('./queries/widgets');
...
app.post('/api/widgets', widgets.create)
app.get('/api/widgets', widgets.retrieve)
app.get('/api/widgets/:id', widgets.retrieve)
app.put('/api/widgets/:id', widgets.update)
app.delete('/api/widgets/:id', widgets.remove);

Create queries folder in the root of your project with widgets.js inside:

mkdir queries
touch queries/widgets.js

Let’s start with the code that retrieves all widgets from the database. any() indicates we expect any number of results; it returns a promise:

db.any('select * from widgets')
  .then((results) => {
    res.status(200)
      .json({
        status: 'success',
        results: results,
        message: 'All widgets retrieved'
      })
  })
  .catch((err) => next(err));

It’s very similar to retrieve a single widget

db.one('select * from widgets where id = $1', widgetId)
  .then((results) => {
    res.status(200)
      .json({
        status: 'success',
        results: results,
        message: `Retrieved single widget with ID = ${widgetId}`
      });
  })
  .catch((err) => next(err));

Let’s combine that into a retrieve() function:

function retrieve(req, res, next) {
  if (req.params.id) {
    db.one(...)
  } else {
    db.any(...)
  }
}

Create function uses .none() with some additional pre-processing:

function create(req, res, next) {
  const amount = parseInt(req.body.amount);
  const name = req.body.name;

  db.none('insert into widgets(name, amount)' +
          'values(${name}, ${amount})', { name, amount })
    .then(() => {
      res.status(201)
        .json({
          status: 'success',
          message: 'Widget successfully created'
        });
    })
    .catch((err) => next(err));
}

For update() function you must always specify all parameters. It is PUT in HTTP parlance.

function update(req, res, next) {
  const { name, amount } = req.body;

  db.none('update widgets set name=$1, amount=$2 where id=$3',
    [name, parseInt(amount), parseInt(req.params.id)])
    .then(() => {
      res.status(200)
        .json({
          status: 'success',
          message: 'Widget successfully updated'
        });
    })
    .catch((err) => next(err));}

Finally, remove() function:

function remove(req, res, next) {
  const widgetId = parseInt(req.params.id);

  db.result('delete from widgets where id = $1', widgetId)
    .then((results) => {
      res.status(200)
        .json({
          status: 'success',
          message: `Widget with ID = ${widgetId} successfully deleted.`
        });
    })
    .catch((err) => next(err));
}

Put all those functions inside queries/widgets.js. At the top of the file add the following:

const pg = require('pg-promise')()
const db = pg('postgres://localhost:5432/widgets')

module.exports = { create, retrieve, update, remove }

Let’s test it. First retrieve all elements

http :3000/api/widgets

Retrieve a single widget

http :3000/api/widgets/3

Create a new widget

http :3000/api/widgets name="Widget 4" amount=234 --form

We're parsing request's body using urlencoded format, that's why I used --form flag. Usually we use JSON to communicate with an API. Simply add bodyParser.json() to indicate that request's body is in JSON format.

Update an widget (all fields must be specified)

http PUT :3000/api/widgets/4 name="Widget 4" amount=999 --form

Finally, delete a widget

http DELETE :3000/api/widgets/4

These are just basic elements for building a web application which provides a REST-like API to interact with widgets. You can now install Express Generator to automatically recreate most of it, but with a better understanding of the underlaying principles.

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