Built-in Middleware
Express comes with several built-in middleware functions that handle common tasks. These are included out of the box, so you don't need to install additional packages. The most commonly used are `express.json()` and `express.urlencoded()` for parsing request bodies.
express.json()
This middleware parses incoming requests with JSON payloads. It's essential for building APIs that accept JSON data.
const express = require('express');const app = express();
<!-- Parse JSON bodies -->app.use(express.json());
app.post('/api/users', (req, res) => { console.log(req.body); <!-- Now contains parsed JSON --> res.json({ received: req.body });});Without `express.json()`, `req.body` would be `undefined` or the raw request stream.
express.urlencoded()
This middleware parses incoming requests with URL-encoded payloads (the format sent by HTML forms with `method="POST"`).
<!-- Parse URL-encoded bodies (form data) -->app.use(express.urlencoded({ extended: true }));
app.post('/submit-form', (req, res) => { console.log(req.body); <!-- { name: 'John', email: 'john@example.com' } --> res.send('Form submitted');});extended: true vs false
| Option | Description |
|---|---|
extended: true | Parses nested objects using the `qs` library (more powerful). |
extended: false | Parses using the `querystring` library (simpler, flat objects). |
express.static()
This built-in middleware serves static files like HTML, CSS, images, and client-side JavaScript.
<!-- Serve files from the 'public' folder -->app.use(express.static('public'));
<!-- Now files in public/ are accessible: --><!-- http://localhost:3000/css/style.css --><!-- http://localhost:3000/images/logo.png -->Combining Built-in Middleware
const express = require('express');const app = express();
<!-- Parse JSON and form data -->app.use(express.json());app.use(express.urlencoded({ extended: true }));
<!-- Serve static files -->app.use(express.static('public'));
<!-- Now your app can handle JSON APIs, form submissions, and static files -->Two Minute Drill
- `express.json()` parses incoming JSON requests – essential for REST APIs.
- `express.urlencoded()` parses form submissions (URL-encoded data).
- `express.static()` serves static files from a directory.
- These are built-in – no need for additional npm packages.
- Always place body-parsing middleware before routes that need the data.
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