Express.js Cheatsheet — goDevelopers

Setup

install

npm init -y
npm install express
npm install -D nodemon

index.js

const express = require('express')
const app = express()
app.use(express.json())
app.listen(3000)

HTTP Methods

GET	Read data
POST	Create data
PUT	Replace data
PATCH	Partial update
DELETE	Remove data

Status Codes

200OK

201Created

204No Content

301Moved

400Bad Request

401Unauthorized

403Forbidden

404Not Found

500Server Error

503Unavailable

Route Patterns

// Basic routes
app.get('/users', (req, res) => res.json({ users: [] }))
app.post('/users', (req, res) => res.status(201).json(req.body))

// URL param  →  /users/42
app.get('/users/:id', (req, res) => {
  const { id } = req.params   // "42"
  res.json({ id })
})

// Query string  →  /search?q=node&limit=5
app.get('/search', (req, res) => {
  const { q, limit } = req.query
  res.json({ q, limit })
})

req & res

req.body	POST payload
req.params	URL :id values
req.query	?key=val pairs
req.headers	HTTP headers
res.json()	Send JSON
res.status()	Set status code
res.send()	Send string/buffer
res.redirect()	Redirect URL

Middleware

Runs between request → route handler. Must call next() or the request hangs forever.

// Custom logger middleware
app.use((req, res, next) => {
  console.log(`${req.method} ${req.url}`)
  next()   // pass to next handler
})

// Apply only to specific routes
app.get('/admin', authMiddleware, (req, res) => { ... })

Request

→

Middleware 1

→

Middleware 2

→

Route Handler

→

Response

Common Middleware

express.json()	Parse JSON body
express.urlencoded()	Parse form data
express.static()	Serve static files
cors	Cross-origin requests
helmet	Security headers
morgan	HTTP request logger
dotenv	Load .env variables

DB Connect Before Server Start

const mongoose = require('mongoose')

mongoose.connect(process.env.MONGO_URI)
  .then(() => {
    console.log('DB connected')
    app.listen(3000, () => console.log('Server running'))
  })
  .catch((err) => {
    console.error('DB failed', err)
    process.exit(1)  // crash loudly — no DB = no point running
  })

Error Handling

// In route — catch & pass
app.get('/data', async (req, res, next) => {
  try {
    const d = await fetchData()
    res.json(d)
  } catch (err) { next(err) }
})

// Error handler — 4 args, placed LAST
app.use((err, req, res, next) => {
  res.status(500).json({ error: err.message })
})

Router (file splitting)

// routes/users.js
const r = require('express').Router()
r.get('/', ...)
r.post('/', ...)
module.exports = r

// index.js
app.use('/users', require('./routes/users'))

Folder Structure

project/
├── routes/
│   ├── users.js
│   └── posts.js
├── middleware/
│   └── auth.js
├── controllers/
│   └── userController.js
├── .env
├── .gitignore
└── index.js

Env Variables

.env file

PORT=3000
MONGO_URI=mongodb://...
JWT_SECRET=supersecret

usage

require('dotenv').config()
const port = process.env.PORT
// NEVER commit .env to git

Biggest Beginner Mistakes — Hustlers Hub Warning Board

Forgetting express.json() Body is undefined on every POST. Add it before all routes.

Not calling next(err) in async Errors crash silently. Wrap in try/catch, pass err to next().

Sending response twice res.json() then res.send() = "headers already sent". One response only.

Error handler needs 4 args Express ignores it as error middleware if you use only 3 arguments.

Committing .env to Git Add .env to .gitignore immediately. Leaked secrets = major breach.

Test with Postman / Thunder Don't test APIs in a browser. GET works, POST/PUT/DELETE won't.