Expressjs Cheatsheet

December 21, 2022

Setup

mkdir app-name
cd myapp

npm init

entry point: (index.js)

npm install express

npm install express --no-save

In index.js

const express = require('express')
const app = express()
const port = 3000

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

app.listen(port, () => {
  console.log(`Example app listening on port ${port}`)
})

Run app

node app.js

Express application generator

npx express-generator

or 

npm install -g express-generator
express

cd myapp
npm install

Basic routing

app.METHOD(PATH, HANDLER)

The following examples illustrate defining simple routes.

Respond with Hello World! on the homepage:

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

Respond to POST request on the root route (/), the application’s home page:

app.post('/', (req, res) => {
  res.send('Got a POST request')
})

Respond to a PUT request to the /user route:

app.put('/user', (req, res) => {
  res.send('Got a PUT request at /user')
})

Respond to a DELETE request to the /user route:

app.delete('/user', (req, res) => {
  res.send('Got a DELETE request at /user')
})

Serving static files in Express

express.static(root, [options])

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

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

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

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

const path = require('path')
app.use('/static', express.static(path.join(__dirname, 'public')))

Examples -

express/examples at master · expressjs/express

Middleware

Middleware functions are functions that have access to the request object (req), the response object (res), and the next middleware function in the application’s request-response cycle. The next middleware function is commonly denoted by a variable named next.

Example of a simple middleware function

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

const myLogger = (req, res, next) => {
  console.log('LOGGED')
  next()
}

app.use(myLogger)

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

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

Error Handling

Error-handling middleware functions are defined in the same way as other middleware functions, except they have four arguments instead of three: (err, req, res, next).

Example of an error-handling middleware function

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

app.get('/', (req, res) => {
  throw new Error('BROKEN') // Express will catch this on its own.
})

app.use((err, req, res, next) => {
  console.error(err.stack)
  res.status(500).send('Something broke!')
})

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

Using Template Engines

Express supports many template engines. Below is an example of using Pug as the template engine.

Example of setting up Pug

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

app.set('view engine', 'pug')

app.get('/', (req, res) => {
  res.render('index', { title: 'Hey', message: 'Hello there!' })
})

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

Handling Forms

Express can handle form submissions using the body-parser middleware.

Example of handling form data

const express = require('express')
const bodyParser = require('body-parser')
const app = express()

app.use(bodyParser.urlencoded({ extended: false }))

app.post('/submit-form', (req, res) => {
  res.send(`Hello ${req.body.name}`)
})

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

Database Integration

Express can be integrated with databases like MongoDB using libraries like Mongoose.

Example of connecting to MongoDB with Mongoose

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

mongoose.connect('mongodb://localhost/test', { useNewUrlParser: true, useUnifiedTopology: true })

const Cat = mongoose.model('Cat', { name: String })

app.get('/add-cat', (req, res) => {
  const kitty = new Cat({ name: 'Zildjian' })
  kitty.save().then(() => res.send('Cat added!'))
})

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