How does body parsing work in Express.js?

Body Parsing in Express.js

When a client sends data (JSON, form, file), it arrives as a stream of bytes in the request body. Express's built-in body parsers read and parse this data, making it available on req.body.

Built-in Parsers (Express 4.16+)

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

// Parse JSON bodies: { "name": "Alice" }
app.use(express.json());

// Parse URL-encoded form bodies: name=Alice&age=30
app.use(express.urlencoded({ extended: true }));

No extra packages needed — these are built into Express 4.16+.

express.json()

Parses requests with Content-Type: application/json.

app.use(express.json({ limit: '10mb' })); // increase size limit

app.post('/users', (req, res) => {
  console.log(req.body); // { name: 'Alice', email: 'alice@example.com' }
  res.status(201).json(req.body);
});

Client must send: Content-Type: application/json header.

express.urlencoded()

Parses HTML form submissions (Content-Type: application/x-www-form-urlencoded).

app.use(express.urlencoded({ extended: true }));
// extended: true  → uses 'qs' library (nested objects)
// extended: false → uses 'querystring' (flat only)

app.post('/login', (req, res) => {
  const { username, password } = req.body;
  // ...
});

Raw & Text Parsers

// Parse raw binary body
app.use(express.raw({ type: 'application/octet-stream' }));

// Parse plain text body
app.use(express.text({ type: 'text/plain' }));

File Uploads with Multer

For multipart/form-data (file uploads), use the multer package:

bash

npm install multer

const multer = require('multer');

const storage = multer.diskStorage({
  destination: 'uploads/',
  filename: (req, file, cb) => {
    cb(null, Date.now() + '-' + file.originalname);
  }
});

const upload = multer({ storage, limits: { fileSize: 5 * 1024 * 1024 } }); // 5MB

app.post('/upload', upload.single('avatar'), (req, res) => {
  console.log(req.file);  // uploaded file info
  console.log(req.body);  // other form fields
  res.json({ filename: req.file.filename });
});

// Multiple files
app.post('/photos', upload.array('photos', 5), (req, res) => {
  console.log(req.files); // array of files
});

Route-Specific Parsing

Apply parsers only to specific routes:

// Only parse JSON for API routes
app.use('/api', express.json());

// Different limits per route
app.post('/webhook',
  express.raw({ type: 'application/json' }), // raw for Stripe webhook verification
  handleWebhook
);

Common Mistakes

// ❌ Forgot to add body parser
app.post('/users', (req, res) => {
  console.log(req.body); // undefined!
});

// ✅ Add BEFORE route definitions
app.use(express.json());
app.post('/users', (req, res) => {
  console.log(req.body); // { name: 'Alice' }
});

Summary

Use express.json() for API endpoints and express.urlencoded() for HTML forms. Add these middleware at the top of your app, before route definitions. For file uploads, use the multer package.

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