Documentation Index
Fetch the complete documentation index at: https://mintlify.com/expressjs/express/llms.txt
Use this file to discover all available pages before exploring further.
The app object represents the Express application. Create it by calling the top-level express() function.
const express = require('express');
const app = express();
Properties
app.locals
An object containing local variables scoped to the application. These variables are available in all rendered templates.
Local variables available to all views rendered by the application
app.locals.title = 'My App';
app.locals.email = 'support@example.com';
// Available in templates
// <%= title %> - outputs "My App"
app.mountpath
Contains the path pattern(s) on which a sub-app was mounted.
The path or array of paths on which the app was mounted
const express = require('express');
const app = express();
const admin = express();
admin.get('/', (req, res) => {
console.log(admin.mountpath); // '/admin'
res.send('Admin Homepage');
});
app.use('/admin', admin);
Events
mount
The sub-app will emit a ‘mount’ event when mounted on a parent app.
const admin = express();
admin.on('mount', (parent) => {
console.log('Admin Mounted');
console.log(parent); // refers to the parent app
});
app.use('/admin', admin);
Methods
app.all(path, callback [, callback …])
Routes HTTP requests to the specified path with all HTTP methods (GET, POST, PUT, DELETE, etc.).
app.all('/secret', (req, res, next) => {
console.log('Accessing the secret section...');
next(); // pass control to the next handler
});
app.all('/api/*', requireAuthentication);
app.delete(path, callback [, callback …])
Routes HTTP DELETE requests to the specified path.
app.delete('/user/:id', (req, res) => {
res.send(`Delete user ${req.params.id}`);
});
app.disable(name)
Sets the boolean setting name to false.
app.disable('x-powered-by');
app.disabled('x-powered-by'); // true
app.disabled(name)
Returns true if the boolean setting name is disabled (false).
app.disabled('trust proxy'); // false
app.disable('trust proxy');
app.disabled('trust proxy'); // true
app.enable(name)
Sets the boolean setting name to true.
app.enable('trust proxy');
app.enabled('trust proxy'); // true
app.enabled(name)
Returns true if the setting name is enabled (true).
app.enabled('trust proxy'); // false
app.enable('trust proxy');
app.enabled('trust proxy'); // true
app.engine(ext, callback)
Registers a template engine callback as ext. By default, Express will require() the engine based on the file extension.
app.engine('pug', require('pug').__express);
app.engine('html', require('ejs').renderFile);
app.get(name)
Returns the value of the app setting name.
app.get('title'); // undefined
app.set('title', 'My Site');
app.get('title'); // "My Site"
app.get(path, callback [, callback …])
Routes HTTP GET requests to the specified path.
app.get('/', (req, res) => {
res.send('GET request to homepage');
});
app.get('/user/:id', (req, res) => {
res.send(`User ${req.params.id}`);
});
app.listen([port[, host[, backlog]]][, callback])
Binds and listens for connections on the specified host and port. This method is identical to Node’s http.Server.listen().
const server = app.listen(3000, () => {
console.log('Server running on port 3000');
});
// Listening on specific host
app.listen(3000, 'localhost', () => {
console.log('Server running on localhost:3000');
});
app.METHOD(path, callback [, callback …])
Routes HTTP requests where METHOD is the HTTP method (GET, POST, PUT, PATCH, DELETE, etc.) in lowercase.
// HTTP POST method
app.post('/', (req, res) => {
res.send('POST request to homepage');
});
// HTTP PUT method
app.put('/user', (req, res) => {
res.send('PUT request to /user');
});
// HTTP PATCH method
app.patch('/user/:id', (req, res) => {
res.send(`PATCH request to user ${req.params.id}`);
});
checkout, copy, delete, get, head, lock, merge, mkactivity, mkcol, move, m-search, notify, options, patch, post, purge, put, report, search, subscribe, trace, unlock, unsubscribe.
app.param([name], callback)
Adds callback triggers to route parameters. The name can be a single parameter name or an array of names.
app.param('user', (req, res, next, id) => {
// Find user by ID
User.find(id, (err, user) => {
if (err) {
next(err);
} else if (user) {
req.user = user;
next();
} else {
next(new Error('User not found'));
}
});
});
// Multiple parameters
app.param(['id', 'page'], (req, res, next, value) => {
console.log('Parameter:', value);
next();
});
app.path()
Returns the canonical path of the app as a string.
const app = express();
const blog = express();
const blogAdmin = express();
app.use('/blog', blog);
blog.use('/admin', blogAdmin);
console.log(app.path()); // ''
console.log(blog.path()); // '/blog'
console.log(blogAdmin.path()); // '/blog/admin'
app.post(path, callback [, callback …])
Routes HTTP POST requests to the specified path.
app.post('/', (req, res) => {
res.send('POST request to homepage');
});
app.post('/user', (req, res) => {
res.send('Create new user');
});
app.put(path, callback [, callback …])
Routes HTTP PUT requests to the specified path.
app.put('/user/:id', (req, res) => {
res.send(`Update user ${req.params.id}`);
});
app.render(view, [locals], callback)
Returns the rendered HTML of a view via the callback function. Accepts an optional parameter that is an object containing local variables for the view.
app.render('email', { name: 'Tobi' }, (err, html) => {
if (err) {
console.error(err);
return;
}
console.log(html);
// Send email with rendered HTML
});
app.route(path)
Returns an instance of a single route which you can then use to handle HTTP verbs with optional middleware. Use app.route() to avoid duplicate route naming and typos.
app.route('/events')
.get((req, res) => {
res.send('Get all events');
})
.post((req, res) => {
res.send('Create an event');
})
.put((req, res) => {
res.send('Update events');
});
app.set(name, value)
Assigns setting name to value. You may store any value, but certain names can be used to configure the server’s behavior.
app.set('title', 'My Site');
app.set('email', 'support@example.com');
// Configuration settings
app.set('view engine', 'pug');
app.set('views', './views');
Application Settings
Environment
Views
Networking
Response
Routing
env - Environment mode (default: process.env.NODE_ENV or “development”)view cache - Enables view template compilation caching (enabled in production by default)
view engine - The default template engine extension to use when omittedviews - Directory or array of directories for the application’s views (default: process.cwd() + '/views')
trust proxy - Indicates the app is behind a front-facing proxy (default: false)subdomain offset - Number of dot-separated parts to remove to access subdomain (default: 2)
etag - Set ETag response header (default: 'weak', can be 'strong', true, false, or function)x-powered-by - Enables “X-Powered-By: Express” header (default: true)jsonp callback name - Specifies the default JSONP callback name (default: "callback")json escape - Enable escaping JSON responses from res.json, res.jsonp, and res.sendjson replacer - JSON replacer callback for JSON.stringifyjson spaces - JSON spaces for formatting (development default: 2, production: 0)
case sensitive routing - Enable case sensitivity (default: false)strict routing - Enable strict routing (treats /foo and /foo/ differently) (default: false)query parser - Query string parsing library to use: 'simple' (querystring), 'extended' (qs), false, or custom function (default: 'simple')
app.use([path,] callback [, callback…])
Mounts middleware function(s) at the specified path. If path is not specified, it defaults to ”/”.
// Application-level middleware
app.use((req, res, next) => {
console.log('Time:', Date.now());
next();
});
// Middleware for specific path
app.use('/user/:id', (req, res, next) => {
console.log('Request Type:', req.method);
next();
});
// Mount a router
const router = express.Router();
app.use('/api', router);
// Mount an Express app
const admin = express();
app.use('/admin', admin);
// Error-handling middleware (must have 4 arguments)
app.use((err, req, res, next) => {
console.error(err.stack);
res.status(500).send('Something broke!');
});
Complete Example
const express = require('express');
const app = express();
// Settings
app.set('view engine', 'pug');
app.set('views', './views');
app.set('trust proxy', true);
// Locals
app.locals.title = 'My Application';
app.locals.strftime = require('strftime');
// Middleware
app.use(express.json());
app.use(express.urlencoded({ extended: true }));
app.use(express.static('public'));
// Parameter middleware
app.param('user', (req, res, next, id) => {
User.find(id, (err, user) => {
if (err) return next(err);
req.user = user;
next();
});
});
// Routes
app.get('/', (req, res) => {
res.render('index');
});
app.route('/users')
.get((req, res) => res.json(users))
.post((req, res) => res.json({ created: true }));
// Start server
app.listen(3000, () => {
console.log('Server running on port 3000');
});