Built-in middleware

The request body is the data a client sends along with POST, PUT or PATCH: form fields or JSON. Express does not read it by itself, so as not to waste time on requests that do not need a body. Express has ready middleware for parsing built in — nothing to install.

Function	What it parses	`Content-Type` header
`express.json()`	JSON	`application/json`
`express.urlencoded()`	HTML form data	`application/x-www-form-urlencoded`
`express.text()`	Plain text	`text/plain`
`express.static()`	Does not parse, serves files from a folder	—

Each function looks at the Content-Type header and parses only its own format. The result goes into req.body.

JSON

Enable JSON parsing at the top of src/app.js:

// src/app.js
app.use(express.json());
 
app.post('/api/posts', (req, res) => {
  res.status(201).json({ received: req.body });
});

Send a request from the terminal:

curl -X POST http://localhost:3000/api/posts -H "Content-Type: application/json" -d '{"title":"New post"}'

The server returns {"received":{"title":"New post"}}: the body has been parsed into an ordinary object. If the JSON is malformed, Express answers with code 400 by itself.

Form data

An ordinary HTML form sends data in a different format — as a string title=...&body=.... It needs a second middleware:

// src/app.js
app.use(express.urlencoded());
 
app.post('/posts', (req, res) => {
  res.send(`Title: ${req.body.title}`);
});

Now a form with a field name="title" and method="post" passes the value into req.body.title. Forms are covered in detail in the chapter about pages.

Size limit

By default a body can be up to 100 KB — a bigger request gets a 413 response. The limit is changed with the limit option:

// src/app.js
app.use(express.json({ limit: '1mb' }));

Do not raise it without need: a big limit lets a single client send the server megabytes of junk.

Where to add them

Both middleware are usually added as a pair at the top of src/app.js, before any routes:

// src/app.js
const app = express();
 
app.use(express.json());
app.use(express.urlencoded());
 
app.use('/posts', postsRouter);

Common mistake

If the client did not send the Content-Type: application/json header, express.json() skips the body and req.body stays undefined. In Express 5 this is the default value, so req.body.title crashes with Cannot read properties of undefined. Always set the header in curl and fetch(), and read fields carefully in the handler: req.body?.title.

Thanks for reading!

Built-in middleware