express()

Creates an Express application. The express() function is a top-level function exported by the express module.

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

Methods

express.json([options])

This is a built-in middleware function in Express. It parses incoming requests with JSON payloads and is based on body-parser.

Returns middleware that only parses JSON and only looks at requests where the Content-Type header matches the type option. This parser accepts any Unicode encoding of the body and supports automatic inflation of gzip and deflate encodings.

A new body object containing the parsed data is populated on the request object after the middleware (i.e. req.body), or an empty object ({}) if there was no body to parse, the Content-Type was not matched, or an error occurred.

The following table describes the properties of the optional options object.

express.static(root, [options])

This is a built-in middleware function in Express. It serves static files and is based on serve-static.

The root argument specifies the root directory from which to serve static assets. The function determines the file to serve will by combining req.url with the provided root directory. When a file is not found, instead of sending a 404 response, it instead calls next() to move on to the next middleware, allowing for stacking and fall-backs.

The following table describes the properties of the options object. See also the example below.

For more information, see Serving static files in Express. and Using middleware - Built-in middleware.

dotfiles

Possible values for this option are:

• “allow” - No special treatment for dotfiles.
• “deny” - Deny a request for a dotfile, respond with 403, then call next().
• “ignore” - Act as if the dotfile does not exist, respond with 404, then call next().

NOTE: With the default value, it will not ignore files in a directory that begins with a dot.

fallthrough

When this option is true, client errors such as a bad request or a request to a non-existent file will cause this middleware to simply call next() to invoke the next middleware in the stack. When false, these errors (even 404s), will invoke next(err).

Set this option to true so you can map multiple physical directories to the same web address or for routes to fill in non-existent files.

Use false if you have mounted this middleware at a path designed to be strictly a single file system directory, which allows for short-circuiting 404s for less overhead. This middleware will also reply to all methods.

setHeaders

For this option, specify a function to set custom response headers. Alterations to the headers must occur synchronously.

The signature of the function is:

fn(res, path, stat)

Arguments:

• res, the response object.
• path, the file path that is being sent.
• stat, the stat object of the file that is being sent.

Example of express.static

Here is an example of using the express.static middleware function with an elaborate options object:

var options = {
  dotfiles: 'ignore',
  etag: false,
  extensions: ['htm', 'html'],
  index: false,
  maxAge: '1d',
  redirect: false,
  setHeaders: function (res, path, stat) {
    res.set('x-timestamp', Date.now())
  }
}

app.use(express.static('public', options))

express.Router([options])

Creates a new router object.

var router = express.Router([options]);

The optional options parameter specifies the behavior of the router.

You can add middleware and HTTP method routes (such as get, put, post, and so on) to router just like an application.

For more information, see Router.

express.urlencoded([options])

This is a built-in middleware function in Express. It parses incoming requests with urlencoded payloads and is based on body-parser.

Returns middleware that only parses urlencoded bodies and only looks at requests where the Content-Type header matches the type option. This parser accepts only UTF-8 encoding of the body and supports automatic inflation of gzip and deflate encodings.

A new body object containing the parsed data is populated on the request object after the middleware (i.e. req.body), or an empty object ({}) if there was no body to parse, the Content-Type was not matched, or an error occurred. This object will contain key-value pairs, where the value can be a string or array (when extended is false), or any type (when extended is true).

The following table describes the properties of the optional options object.

Application

The app object conventionally denotes the Express application. Create it by calling the top-level express() function exported by the Express module:

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

app.get('/', function(req, res){
  res.send('hello world');
});

app.listen(3000);

The app object has methods for

• Routing HTTP requests; see for example, app.METHOD and app.param.
• Configuring middleware; see app.route.
• Rendering HTML views; see app.render.
• Registering a template engine; see app.engine.

It also has settings (properties) that affect how the application behaves; for more information, see Application settings.

Properties

app.locals

The app.locals object has properties that are local variables within the application.

app.locals.title
// => 'My App'

app.locals.email
// => '[email protected]'

Once set, the value of app.locals properties persist throughout the life of the application, in contrast with res.locals properties that are valid only for the lifetime of the request.

You can access local variables in templates rendered within the application. This is useful for providing helper functions to templates, as well as application-level data. Local variables are available in middleware via req.app.locals (see req.app)

app.locals.title = 'My App';
app.locals.strftime = require('strftime');
app.locals.email = '[email protected]';

app.mountpath

The app.mountpath property contains one or more path patterns on which a sub-app was mounted.

var express = require('express');

var app = express(); // the main app
var admin = express(); // the sub app

admin.get('/', function (req, res) {
  console.log(admin.mountpath); // /admin
  res.send('Admin Homepage');
});

app.use('/admin', admin); // mount the sub app

It is similar to the baseUrl property of the req object, except req.baseUrl returns the matched URL path, instead of the matched patterns.

If a sub-app is mounted on multiple path patterns, app.mountpath returns the list of patterns it is mounted on, as shown in the following example.

var admin = express();

admin.get('/', function (req, res) {
  console.log(admin.mountpath); // [ '/adm*n', '/manager' ]
  res.send('Admin Homepage');
});

var secret = express();
secret.get('/', function (req, res) {
  console.log(secret.mountpath); // /secr*t
  res.send('Admin Secret');
});

admin.use('/secr*t', secret); // load the 'secret' router on '/secr*t', on the 'admin' sub app
app.use(['/adm*n', '/manager'], admin); // load the 'admin' router on '/adm*n' and '/manager', on the parent app

Events

app.on('mount', callback(parent))

The mount event is fired on a sub-app, when it is mounted on a parent app. The parent app is passed to the callback function.

var admin = express();

admin.on('mount', function (parent) {
  console.log('Admin Mounted');
  console.log(parent); // refers to the parent app
});

admin.get('/', function (req, res) {
  res.send('Admin Homepage');
});

app.use('/admin', admin);

Methods

app.all(path, callback [, callback ...])

This method is like the standard app.METHOD() methods, except it matches all HTTP verbs.

Arguments

Examples

The following callback is executed for requests to /secret whether using GET, POST, PUT, DELETE, or any other HTTP request method:

app.all('/secret', function (req, res, next) {
  console.log('Accessing the secret section ...')
  next() // pass control to the next handler
});

The app.all() method is useful for mapping “global” logic for specific path prefixes or arbitrary matches. For example, if you put the following at the top of all other route definitions, it requires that all routes from that point on require authentication, and automatically load a user. Keep in mind that these callbacks do not have to act as end-points: loadUser can perform a task, then call next() to continue matching subsequent routes.

app.all('*', requireAuthentication, loadUser);

Or the equivalent:

app.all('*', requireAuthentication);
app.all('*', loadUser);

Another example is white-listed “global” functionality. The example is similar to the ones above, but it only restricts paths that start with “/api”:

app.all('/api/*', requireAuthentication);

app.delete(path, callback [, callback ...])

Routes HTTP DELETE requests to the specified path with the specified callback functions. For more information, see the routing guide.

Arguments

Example

app.delete('/', function (req, res) {
  res.send('DELETE request to homepage');
});

app.disable(name)

Sets the Boolean setting name to false, where name is one of the properties from the app settings table. Calling app.set('foo', false) for a Boolean property is the same as calling app.disable('foo').

For example:

app.disable('trust proxy');
app.get('trust proxy');
// => false

app.disabled(name)

Returns true if the Boolean setting name is disabled (false), where name is one of the properties from the app settings table.

app.disabled('trust proxy');
// => true

app.enable('trust proxy');
app.disabled('trust proxy');
// => false

app.enable(name)

Sets the Boolean setting name to true, where name is one of the properties from the app settings table. Calling app.set('foo', true) for a Boolean property is the same as calling app.enable('foo').

app.enable('trust proxy');
app.get('trust proxy');
// => true

app.enabled(name)

Returns true if the setting name is enabled (true), where name is one of the properties from the app settings table.

app.enabled('trust proxy');
// => false

app.enable('trust proxy');
app.enabled('trust proxy');
// => true

app.engine(ext, callback)

Registers the given template engine callback as ext.

By default, Express will require() the engine based on the file extension. For example, if you try to render a “foo.pug” file, Express invokes the following internally, and caches the require() on subsequent calls to increase performance.

app.engine('pug', require('pug').__express);

Use this method for engines that do not provide .__express out of the box, or if you wish to “map” a different extension to the template engine.

For example, to map the EJS template engine to “.html” files:

app.engine('html', require('ejs').renderFile);

In this case, EJS provides a .renderFile() method with the same signature that Express expects: (path, options, callback), though note that it aliases this method as ejs.__express internally so if you’re using “.ejs” extensions you don’t need to do anything.

Some template engines do not follow this convention. The consolidate.js library maps Node template engines to follow this convention, so they work seamlessly with Express.

var engines = require('consolidate');
app.engine('haml', engines.haml);
app.engine('html', engines.hogan);

app.get(name)

Returns the value of name app setting, where name is one of the strings in the app settings table. For example:

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 with the specified callback functions.

Arguments

For more information, see the routing guide.

Example

app.get('/', function (req, res) {
  res.send('GET request to homepage');
});

app.listen(path, [callback])

Starts a UNIX socket and listens for connections on the given path. This method is identical to Node’s http.Server.listen().

var express = require('express');
var app = express();
app.listen('/tmp/sock');

app.listen(port, [hostname], [backlog], [callback])

Binds and listens for connections on the specified host and port. This method is identical to Node’s http.Server.listen().

var express = require('express');
var app = express();
app.listen(3000);

The app returned by express() is in fact a JavaScript Function, designed to be passed to Node’s HTTP servers as a callback to handle requests. This makes it easy to provide both HTTP and HTTPS versions of your app with the same code base, as the app does not inherit from these (it is simply a callback):

var express = require('express');
var https = require('https');
var http = require('http');
var app = express();

http.createServer(app).listen(80);
https.createServer(options, app).listen(443);

The app.listen() method returns an http.Server object and (for HTTP) is a convenience method for the following:

app.listen = function() {
  var server = http.createServer(this);
  return server.listen.apply(server, arguments);
};

app.METHOD(path, callback [, callback ...])

Routes an HTTP request, where METHOD is the HTTP method of the request, such as GET, PUT, POST, and so on, in lowercase. Thus, the actual methods are app.get(), app.post(), app.put(), and so on. See Routing methods below for the complete list.

Arguments

Routing methods

Express supports the following routing methods corresponding to the HTTP methods of the same names:

The API documentation has explicit entries only for the most popular HTTP methods app.get(), app.post(), app.put(), and app.delete(). However, the other methods listed above work in exactly the same way.

To route methods that translate to invalid JavaScript variable names, use the bracket notation. For example, app['m-search']('/', function ....

The method, app.all(), is not derived from any HTTP method and loads middleware at the specified path for all HTTP request methods. For more information, see app.all.

For more information on routing, see the routing guide.

app.param([name], callback)

Add callback triggers to route parameters, where name is the name of the parameter or an array of them, and callback is the callback function. The parameters of the callback function are the request object, the response object, the next middleware, the value of the parameter and the name of the parameter, in that order.

If name is an array, the callback trigger is registered for each parameter declared in it, in the order in which they are declared. Furthermore, for each declared parameter except the last one, a call to next inside the callback will call the callback for the next declared parameter. For the last parameter, a call to next will call the next middleware in place for the route currently being processed, just like it would if name were just a string.

For example, when :user is present in a route path, you may map user loading logic to automatically provide req.user to the route, or perform validations on the parameter input.

app.param('user', function(req, res, next, id) {

  // try to get the user details from the User model and attach it to the request object
  User.find(id, function(err, user) {
    if (err) {
      next(err);
    } else if (user) {
      req.user = user;
      next();
    } else {
      next(new Error('failed to load user'));
    }
  });
});

Param callback functions are local to the router on which they are defined. They are not inherited by mounted apps or routers. Hence, param callbacks defined on app will be triggered only by route parameters defined on app routes.

All param callbacks will be called before any handler of any route in which the param occurs, and they will each be called only once in a request-response cycle, even if the parameter is matched in multiple routes, as shown in the following examples.

app.param('id', function (req, res, next, id) {
  console.log('CALLED ONLY ONCE');
  next();
});

app.get('/user/:id', function (req, res, next) {
  console.log('although this matches');
  next();
});

app.get('/user/:id', function (req, res) {
  console.log('and this matches too');
  res.end();
});

On GET /user/42, the following is printed:

CALLED ONLY ONCE
although this matches
and this matches too

app.param(['id', 'page'], function (req, res, next, value) {
  console.log('CALLED ONLY ONCE with', value);
  next();
});

app.get('/user/:id/:page', function (req, res, next) {
  console.log('although this matches');
  next();
});

app.get('/user/:id/:page', function (req, res) {
  console.log('and this matches too');
  res.end();
});

On GET /user/42/3, the following is printed:

CALLED ONLY ONCE with 42
CALLED ONLY ONCE with 3
although this matches
and this matches too

The behavior of the app.param(name, callback) method can be altered entirely by passing only a function to app.param(). This function is a custom implementation of how app.param(name, callback) should behave - it accepts two parameters and must return a middleware.

The first parameter of this function is the name of the URL parameter that should be captured, the second parameter can be any JavaScript object which might be used for returning the middleware implementation.

The middleware returned by the function decides the behavior of what happens when a URL parameter is captured.

In this example, the app.param(name, callback) signature is modified to app.param(name, accessId). Instead of accepting a name and a callback, app.param() will now accept a name and a number.

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

// customizing the behavior of app.param()
app.param(function(param, option) {
  return function (req, res, next, val) {
    if (val == option) {
      next();
    }
    else {
      next('route');
    }
  }
});

// using the customized app.param()
app.param('id', 1337);

// route to trigger the capture
app.get('/user/:id', function (req, res) {
  res.send('OK');
});

app.listen(3000, function () {
  console.log('Ready');
});

In this example, the app.param(name, callback) signature remains the same, but instead of a middleware callback, a custom data type checking function has been defined to validate the data type of the user id.

app.param(function(param, validator) {
  return function (req, res, next, val) {
    if (validator(val)) {
      next();
    }
    else {
      next('route');
    }
  }
});

app.param('id', function (candidate) {
  return !isNaN(parseFloat(candidate)) && isFinite(candidate);
});

//captures '1-a_6' but not '543-azser-sder'
router.get('/[0-9]+-[[\\w]]*', function);

//captures '1-a_6' and '543-az(ser"-sder' but not '5-a s'
router.get('/[0-9]+-[[\\S]]*', function);

//captures all (equivalent to '.*')
router.get('[[\\s\\S]]*', function);

app.path()

Returns the canonical path of the app, a string.

var app = express()
  , blog = express()
  , 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'

The behavior of this method can become very complicated in complex cases of mounted apps: it is usually better to use req.baseUrl to get the canonical path of the app.

app.post(path, callback [, callback ...])

Routes HTTP POST requests to the specified path with the specified callback functions. For more information, see the routing guide.

Arguments

Example

app.post('/', function (req, res) {
  res.send('POST request to homepage');
});

app.put(path, callback [, callback ...])

Routes HTTP PUT requests to the specified path with the specified callback functions.

Arguments

Example

app.put('/', function (req, res) {
  res.send('PUT request to homepage');
});

app.render(view, [locals], callback)

Returns the rendered HTML of a view via the callback function. It accepts an optional parameter that is an object containing local variables for the view. It is like res.render(), except it cannot send the rendered view to the client on its own.

app.render('email', function(err, html){
  // ...
});

app.render('email', { name: 'Tobi' }, function(err, 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 names (and thus typo errors).

var app = express();

app.route('/events')
.all(function(req, res, next) {
  // runs for all HTTP verbs first
  // think of it as route specific middleware!
})
.get(function(req, res, next) {
  res.json(...);
})
.post(function(req, res, next) {
  // maybe add a new event...
});

app.set(name, value)

Assigns setting name to value. You may store any value that you want, but certain names can be used to configure the behavior of the server. These special names are listed in the app settings table.

Calling app.set('foo', true) for a Boolean property is the same as calling app.enable('foo'). Similarly, calling app.set('foo', false) for a Boolean property is the same as calling app.disable('foo').

Retrieve the value of a setting with app.get().

app.set('title', 'My Site');
app.get('title'); // "My Site"

Application Settings

The following table lists application settings.

Note that sub-apps will:

• Not inherit the value of settings that have a default value. You must set the value in the sub-app.
• Inherit the value of settings with no default value; these are explicitly noted in the table below.

Exceptions: Sub-apps will inherit the value of trust proxy even though it has a default value (for backward-compatibility); Sub-apps will not inherit the value of view cache in production (when NODE_ENV is “production”).

app.set('trust proxy', 'loopback') 

app.set('trust proxy', 'loopback, 123.123.123.123') 

app.set('trust proxy', 'loopback, linklocal, uniquelocal') 

app.set('trust proxy', ['loopback', 'linklocal', 'uniquelocal'])

app.set('trust proxy', function (ip) {
  if (ip === '127.0.0.1' || ip === '123.123.123.123') return true; // trusted IPs
  else return false;
});

app.set('etag', function (body, encoding) {
  return generateHash(body, encoding); // consider the function is defined
});

app.use([path,] callback [, callback...])

Mounts the specified middleware function or functions at the specified path: the middleware function is executed when the base of the requested path matches path.

Arguments

Description

A route will match any path that follows its path immediately with a “/”. For example: app.use('/apple', ...) will match “/apple”, “/apple/images”, “/apple/images/news”, and so on.

Since path defaults to “/”, middleware mounted without a path will be executed for every request to the app.
For example, this middleware function will be executed for every request to the app:

app.use(function (req, res, next) {
  console.log('Time: %d', Date.now());
  next();
});

Middleware functions are executed sequentially, therefore the order of middleware inclusion is important.

// this middleware will not allow the request to go beyond it
app.use(function(req, res, next) {
  res.send('Hello World');
});

// requests will never reach this route
app.get('/', function (req, res) {
  res.send('Welcome');
});

Error-handling middleware

Error-handling middleware always takes four arguments. You must provide four arguments to identify it as an error-handling middleware function. Even if you don’t need to use the next object, you must specify it to maintain the signature. Otherwise, the next object will be interpreted as regular middleware and will fail to handle errors. For details about error-handling middleware, see: Error handling.

Define error-handling middleware functions in the same way as other middleware functions, except with four arguments instead of three, specifically with the signature (err, req, res, next)):

app.use(function(err, req, res, next) {
  console.error(err.stack);
  res.status(500).send('Something broke!');
});

Path examples

The following table provides some simple examples of valid path values for mounting middleware.

app.use('/abcd', function (req, res, next) {
  next();
});

app.use('/abc?d', function (req, res, next) {
  next();
});

app.use('/ab+cd', function (req, res, next) {
  next();
});

app.use('/ab\*cd', function (req, res, next) {
  next();
});

app.use('/a(bc)?d', function (req, res, next) {
  next();
});

app.use(/\/abc|\/xyz/, function (req, res, next) {
  next();
});

app.use(['/abcd', '/xyza', /\/lmn|\/pqr/], function (req, res, next) {
  next();
});

Middleware callback function examples

The following table provides some simple examples of middleware functions that can be used as the callback argument to app.use(), app.METHOD(), and app.all(). Even though the examples are for app.use(), they are also valid for app.use(), app.METHOD(), and app.all().

app.use(function (req, res, next) {
  next();
});

var router = express.Router();
router.get('/', function (req, res, next) {
  next();
});
app.use(router);

var subApp = express();
subApp.get('/', function (req, res, next) {
  next();
});
app.use(subApp);

var r1 = express.Router();
r1.get('/', function (req, res, next) {
  next();
});

var r2 = express.Router();
r2.get('/', function (req, res, next) {
  next();
});

app.use(r1, r2);

var r1 = express.Router();
r1.get('/', function (req, res, next) {
  next();
});

var r2 = express.Router();
r2.get('/', function (req, res, next) {
  next();
});

app.use('/', [r1, r2]);

function mw1(req, res, next) { next(); }
function mw2(req, res, next) { next(); }

var r1 = express.Router();
r1.get('/', function (req, res, next) { next(); });

var r2 = express.Router();
r2.get('/', function (req, res, next) { next(); });

var subApp = express();
subApp.get('/', function (req, res, next) { next(); });

app.use(mw1, [mw2, r1, r2], subApp);

Following are some examples of using the express.static middleware in an Express app.

Serve static content for the app from the “public” directory in the application directory:

// GET /style.css etc
app.use(express.static(__dirname + '/public'));

Mount the middleware at “/static” to serve static content only when their request path is prefixed with “/static”:

// GET /static/style.css etc.
app.use('/static', express.static(__dirname + '/public'));

Disable logging for static content requests by loading the logger middleware after the static middleware:

app.use(express.static(__dirname + '/public'));
app.use(logger());

Serve static files from multiple directories, but give precedence to “./public” over the others:

app.use(express.static(__dirname + '/public'));
app.use(express.static(__dirname + '/files'));
app.use(express.static(__dirname + '/uploads'));

Request

The req object represents the HTTP request and has properties for the request query string, parameters, body, HTTP headers, and so on. In this documentation and by convention, the object is always referred to as req (and the HTTP response is res) but its actual name is determined by the parameters to the callback function in which you’re working.

For example:

app.get('/user/:id', function(req, res) {
  res.send('user ' + req.params.id);
});

But you could just as well have:

app.get('/user/:id', function(request, response) {
  response.send('user ' + request.params.id);
});

The req object is an enhanced version of Node’s own request object and supports all built-in fields and methods.

Properties

req.app

This property holds a reference to the instance of the Express application that is using the middleware.

If you follow the pattern in which you create a module that just exports a middleware function and require() it in your main file, then the middleware can access the Express instance via req.app

For example:

//index.js
app.get('/viewdirectory', require('./mymiddleware.js'))

//mymiddleware.js
module.exports = function (req, res) {
  res.send('The views directory is ' + req.app.get('views'));
});

req.baseUrl

The URL path on which a router instance was mounted.

The req.baseUrl property is similar to the mountpath property of the app object, except app.mountpath returns the matched path pattern(s).

For example:

var greet = express.Router();

greet.get('/jp', function (req, res) {
  console.log(req.baseUrl); // /greet
  res.send('Konichiwa!');
});

app.use('/greet', greet); // load the router on '/greet'

Even if you use a path pattern or a set of path patterns to load the router, the baseUrl property returns the matched string, not the pattern(s). In the following example, the greet router is loaded on two path patterns.

app.use(['/gre+t', '/hel{2}o'], greet); // load the router on '/gre+t' and '/hel{2}o'

When a request is made to /greet/jp, req.baseUrl is “/greet”. When a request is made to /hello/jp, req.baseUrl is “/hello”.

req.body

Contains key-value pairs of data submitted in the request body. By default, it is undefined, and is populated when you use body-parsing middleware such as body-parser and multer.

The following example shows how to use body-parsing middleware to populate req.body.

var app = require('express')();
var bodyParser = require('body-parser');
var multer = require('multer'); // v1.0.5
var upload = multer(); // for parsing multipart/form-data

app.use(bodyParser.json()); // for parsing application/json
app.use(bodyParser.urlencoded({ extended: true })); // for parsing application/x-www-form-urlencoded

app.post('/profile', upload.array(), function (req, res, next) {
  console.log(req.body);
  res.json(req.body);
});

req.cookies

When using cookie-parser middleware, this property is an object that contains cookies sent by the request. If the request contains no cookies, it defaults to {}.

// Cookie: name=tj
req.cookies.name
// => "tj"

If the cookie has been signed, you have to use req.signedCookies.

For more information, issues, or concerns, see cookie-parser.

req.fresh

Indicates whether the request is “fresh.” It is the opposite of req.stale.

It is true if the cache-control request header doesn’t have a no-cache directive and any of the following are true:

• The if-modified-since request header is specified and last-modified request header is equal to or earlier than the modified response header.
• The if-none-match request header is *.
• The if-none-match request header, after being parsed into its directives, does not match the etag response header.

req.fresh
// => true

For more information, issues, or concerns, see fresh.

req.hostname

Contains the hostname derived from the Host HTTP header.

When the trust proxy setting does not evaluate to false, this property will instead have the value of the X-Forwarded-Host header field. This header can be set by the client or by the proxy.

// Host: "example.com:3000"
req.hostname
// => "example.com"

req.ip

Contains the remote IP address of the request.

When the trust proxy setting does not evaluate to false, the value of this property is derived from the left-most entry in the X-Forwarded-For header. This header can be set by the client or by the proxy.

req.ip
// => "127.0.0.1"

req.ips

When the trust proxy setting does not evaluate to false, this property contains an array of IP addresses specified in the X-Forwarded-For request header. Otherwise, it contains an empty array. This header can be set by the client or by the proxy.

For example, if X-Forwarded-For is client, proxy1, proxy2, req.ips would be ["client", "proxy1", "proxy2"], where proxy2 is the furthest downstream.

req.method

Contains a string corresponding to the HTTP method of the request: GET, POST, PUT, and so on.

req.originalUrl

This property is much like req.url; however, it retains the original request URL, allowing you to rewrite req.url freely for internal routing purposes. For example, the “mounting” feature of app.use() will rewrite req.url to strip the mount point.

// GET /search?q=something
req.originalUrl
// => "/search?q=something"

In a middleware function, req.originalUrl is a combination of req.baseUrl and req.path, as shown in the following example.

app.use('/admin', function(req, res, next) {  // GET 'http://www.example.com/admin/new'
  console.log(req.originalUrl); // '/admin/new'
  console.log(req.baseUrl); // '/admin'
  console.log(req.path); // '/new'
  next();
});

req.params

This property is an object containing properties mapped to the named route “parameters”. For example, if you have the route /user/:name, then the “name” property is available as req.params.name. This object defaults to {}.

// GET /user/tj
req.params.name
// => "tj"

When you use a regular expression for the route definition, capture groups are provided in the array using req.params[n], where n is the n^th capture group. This rule is applied to unnamed wild card matches with string routes such as /file/*:

// GET /file/javascripts/jquery.js
req.params[0]
// => "javascripts/jquery.js"

If you need to make changes to a key in req.params, use the app.param handler. Changes are applicable only to parameters already defined in the route path.

Any changes made to the req.params object in a middleware or route handler will be reset.

req.path

Contains the path part of the request URL.

// example.com/users?sort=desc
req.path
// => "/users"

req.protocol

Contains the request protocol string: either http or (for TLS requests) https.

When the trust proxy setting does not evaluate to false, this property will use the value of the X-Forwarded-Proto header field if present. This header can be set by the client or by the proxy.

req.protocol
// => "http"

req.query

This property is an object containing a property for each query string parameter in the route. If there is no query string, it is the empty object, {}.

// GET /search?q=tobi+ferret
req.query.q
// => "tobi ferret"

// GET /shoes?order=desc&shoe[color]=blue&shoe[type]=converse
req.query.order
// => "desc"

req.query.shoe.color
// => "blue"

req.query.shoe.type
// => "converse"

req.route

Contains the currently-matched route, a string. For example:

app.get('/user/:id?', function userIdHandler(req, res) {
  console.log(req.route);
  res.send('GET');
});

Example output from the previous snippet:

{ path: '/user/:id?',
  stack:
   [ { handle: [Function: userIdHandler],
       name: 'userIdHandler',
       params: undefined,
       path: undefined,
       keys: [],
       regexp: /^\/?$/i,
       method: 'get' } ],
  methods: { get: true } }

req.secure

A Boolean property that is true if a TLS connection is established. Equivalent to:

'https' == req.protocol;

req.signedCookies

When using cookie-parser middleware, this property contains signed cookies sent by the request, unsigned and ready for use. Signed cookies reside in a different object to show developer intent; otherwise, a malicious attack could be placed on req.cookie values (which are easy to spoof). Note that signing a cookie does not make it “hidden” or encrypted; but simply prevents tampering (because the secret used to sign is private).

If no signed cookies are sent, the property defaults to {}.

// Cookie: user=tobi.CP7AWaXDfAKIRfH49dQzKJx7sKzzSoPq7/AcBBRVwlI3
req.signedCookies.user
// => "tobi"

For more information, issues, or concerns, see cookie-parser.

req.stale

Indicates whether the request is “stale,” and is the opposite of req.fresh. For more information, see req.fresh.

req.stale
// => true

req.subdomains

An array of subdomains in the domain name of the request.

// Host: "tobi.ferrets.example.com"
req.subdomains
// => ["ferrets", "tobi"]

The application property subdomain offset, which defaults to 2, is used for determining the beginning of the subdomain segments. To change this behavior, change its value using app.set.

req.xhr

A Boolean property that is true if the request’s X-Requested-With header field is “XMLHttpRequest”, indicating that the request was issued by a client library such as jQuery.

req.xhr
// => true

Methods

req.accepts(types)

Checks if the specified content types are acceptable, based on the request’s Accept HTTP header field. The method returns the best match, or if none of the specified content types is acceptable, returns false (in which case, the application should respond with 406 "Not Acceptable").

The type value may be a single MIME type string (such as “application/json”), an extension name such as “json”, a comma-delimited list, or an array. For a list or array, the method returns the best match (if any).

// Accept: text/html
req.accepts('html');
// => "html"

// Accept: text/*, application/json
req.accepts('html');
// => "html"
req.accepts('text/html');
// => "text/html"
req.accepts(['json', 'text']);
// => "json"
req.accepts('application/json');
// => "application/json"

// Accept: text/*, application/json
req.accepts('image/png');
req.accepts('png');
// => undefined

// Accept: text/*;q=.5, application/json
req.accepts(['html', 'json']);
// => "json"

For more information, or if you have issues or concerns, see accepts.

req.acceptsCharsets(charset [, ...])

Returns the first accepted charset of the specified character sets, based on the request’s Accept-Charset HTTP header field. If none of the specified charsets is accepted, returns false.

For more information, or if you have issues or concerns, see accepts.

req.acceptsEncodings(encoding [, ...])

Returns the first accepted encoding of the specified encodings, based on the request’s Accept-Encoding HTTP header field. If none of the specified encodings is accepted, returns false.

For more information, or if you have issues or concerns, see accepts.

req.acceptsLanguages(lang [, ...])

Returns the first accepted language of the specified languages, based on the request’s Accept-Language HTTP header field. If none of the specified languages is accepted, returns false.

For more information, or if you have issues or concerns, see accepts.

req.get(field)

Returns the specified HTTP request header field (case-insensitive match). The Referrer and Referer fields are interchangeable.

req.get('Content-Type');
// => "text/plain"

req.get('content-type');
// => "text/plain"

req.get('Something');
// => undefined

Aliased as req.header(field).

req.is(type)

Returns the matching content type if the incoming request’s “Content-Type” HTTP header field matches the MIME type specified by the type parameter. Returns false otherwise.

// With Content-Type: text/html; charset=utf-8
req.is('html');       // => 'html'
req.is('text/html');  // => 'text/html'
req.is('text/*');     // => 'text/*'

// When Content-Type is application/json
req.is('json');              // => 'json'
req.is('application/json');  // => 'application/json'
req.is('application/*');     // => 'application/*'

req.is('html');
// => false

For more information, or if you have issues or concerns, see type-is.

req.param(name [, defaultValue])

Returns the value of param name when present.

// ?name=tobi
req.param('name')
// => "tobi"

// POST name=tobi
req.param('name')
// => "tobi"

// /user/tobi for /user/:name
req.param('name')
// => "tobi"

Lookup is performed in the following order:

• req.params
• req.body
• req.query

Optionally, you can specify defaultValue to set a default value if the parameter is not found in any of the request objects.

req.range(size[, options])

Range header parser.

The size parameter is the maximum size of the resource.

The options parameter is an object that can have the following properties.

An array of ranges will be returned or negative numbers indicating an error parsing.

• -2 signals a malformed header string
• -1 signals an unsatisfiable range

// parse header from request
var range = req.range(1000)

// the type of the range
if (range.type === 'bytes') {
  // the ranges
  range.forEach(function (r) {
    // do something with r.start and r.end
  })
}

Response

The res object represents the HTTP response that an Express app sends when it gets an HTTP request.

In this documentation and by convention, the object is always referred to as res (and the HTTP request is req) but its actual name is determined by the parameters to the callback function in which you’re working.

For example:

app.get('/user/:id', function(req, res){
  res.send('user ' + req.params.id);
});

But you could just as well have:

app.get('/user/:id', function(request, response){
  response.send('user ' + request.params.id);
});

The res object is an enhanced version of Node’s own response object and supports all built-in fields and methods.

Properties

res.app

This property holds a reference to the instance of the Express application that is using the middleware.

res.app is identical to the req.app property in the request object.

res.headersSent

Boolean property that indicates if the app sent HTTP headers for the response.

app.get('/', function (req, res) {
  console.log(res.headersSent); // false
  res.send('OK');
  console.log(res.headersSent); // true
});

res.locals

An object that contains response local variables scoped to the request, and therefore available only to the view(s) rendered during that request / response cycle (if any). Otherwise, this property is identical to app.locals.

This property is useful for exposing request-level information such as the request path name, authenticated user, user settings, and so on.

app.use(function(req, res, next){
  res.locals.user = req.user;
  res.locals.authenticated = ! req.user.anonymous;
  next();
});

Methods

res.append(field [, value])

Appends the specified value to the HTTP response header field. If the header is not already set, it creates the header with the specified value. The value parameter can be a string or an array.

Note: calling res.set() after res.append() will reset the previously-set header value.

res.append('Link', ['<http://localhost/>', '<http://localhost:3000/>']);
res.append('Set-Cookie', 'foo=bar; Path=/; HttpOnly');
res.append('Warning', '199 Miscellaneous warning');

res.attachment([filename])

Sets the HTTP response Content-Disposition header field to “attachment”. If a filename is given, then it sets the Content-Type based on the extension name via res.type(), and sets the Content-Disposition “filename=” parameter.

res.attachment();
// Content-Disposition: attachment

res.attachment('path/to/logo.png');
// Content-Disposition: attachment; filename="logo.png"
// Content-Type: image/png

res.cookie(name, value [, options])

Sets cookie name to value. The value parameter may be a string or object converted to JSON.

The options parameter is an object that can have the following properties.

For example:

res.cookie('name', 'tobi', { domain: '.example.com', path: '/admin', secure: true });
res.cookie('rememberme', '1', { expires: new Date(Date.now() + 900000), httpOnly: true });

The encode option allows you to choose the function used for cookie value encoding. Does not support asynchronous functions.

Example use case: You need to set a domain-wide cookie for another site in your organization. This other site (not under your administrative control) does not use URI-encoded cookie values.

//Default encoding
res.cookie('some_cross_domain_cookie', 'http://mysubdomain.example.com',{domain:'example.com'});
// Result: 'some_cross_domain_cookie=http%3A%2F%2Fmysubdomain.example.com; Domain=example.com; Path=/'

//Custom encoding
res.cookie('some_cross_domain_cookie', 'http://mysubdomain.example.com',{domain:'example.com', encode: String});
// Result: 'some_cross_domain_cookie=http://mysubdomain.example.com; Domain=example.com; Path=/;'

The maxAge option is a convenience option for setting “expires” relative to the current time in milliseconds. The following is equivalent to the second example above.

res.cookie('rememberme', '1', { maxAge: 900000, httpOnly: true });

You can pass an object as the value parameter; it is then serialized as JSON and parsed by bodyParser() middleware.

res.cookie('cart', { items: [1,2,3] });
res.cookie('cart', { items: [1,2,3] }, { maxAge: 900000 });

When using cookie-parser middleware, this method also supports signed cookies. Simply include the signed option set to true. Then res.cookie() will use the secret passed to cookieParser(secret) to sign the value.

res.cookie('name', 'tobi', { signed: true });

Later you may access this value through the req.signedCookie object.

res.clearCookie(name [, options])

Clears the cookie specified by name. For details about the options object, see res.cookie().

res.cookie('name', 'tobi', { path: '/admin' });
res.clearCookie('name', { path: '/admin' });

res.download(path [, filename] [, options] [, fn])

Transfers the file at path as an “attachment”. Typically, browsers will prompt the user for download. By default, the Content-Disposition header “filename=” parameter is path (this typically appears in the browser dialog). Override this default with the filename parameter.

When an error occurs or transfer is complete, the method calls the optional callback function fn. This method uses res.sendFile() to transfer the file.

The optional options argument passes through to the underlying res.sendFile() call, and takes the exact same parameters.

res.download('/report-12345.pdf');

res.download('/report-12345.pdf', 'report.pdf');

res.download('/report-12345.pdf', 'report.pdf', function(err){
  if (err) {
    // Handle error, but keep in mind the response may be partially-sent
    // so check res.headersSent
  } else {
    // decrement a download credit, etc.
  }
});

res.end([data] [, encoding])

Ends the response process. This method actually comes from Node core, specifically the response.end() method of http.ServerResponse.

Use to quickly end the response without any data. If you need to respond with data, instead use methods such as res.send() and res.json().

res.end();
res.status(404).end();

res.format(object)

Performs content-negotiation on the Accept HTTP header on the request object, when present. It uses req.accepts() to select a handler for the request, based on the acceptable types ordered by their quality values. If the header is not specified, the first callback is invoked. When no match is found, the server responds with 406 “Not Acceptable”, or invokes the default callback.

The Content-Type response header is set when a callback is selected. However, you may alter this within the callback using methods such as res.set() or res.type().

The following example would respond with { "message": "hey" } when the Accept header field is set to “application/json” or “*/json” (however if it is “*/*”, then the response will be “hey”).

res.format({
  'text/plain': function(){
    res.send('hey');
  },

  'text/html': function(){
    res.send('<p>hey</p>');
  },

  'application/json': function(){
    res.send({ message: 'hey' });
  },

  'default': function() {
    // log the request and respond with 406
    res.status(406).send('Not Acceptable');
  }
});

In addition to canonicalized MIME types, you may also use extension names mapped to these types for a slightly less verbose implementation:

res.format({
  text: function(){
    res.send('hey');
  },

  html: function(){
    res.send('<p>hey</p>');
  },

  json: function(){
    res.send({ message: 'hey' });
  }
});

res.get(field)

Returns the HTTP response header specified by field. The match is case-insensitive.

res.get('Content-Type');
// => "text/plain"

res.json([body])

Sends a JSON response. This method sends a response (with the correct content-type) that is the parameter converted to a JSON string using JSON.stringify().

The parameter can be any JSON type, including object, array, string, Boolean, or number, and you can also use it to convert other values to JSON, such as null, and undefined (although these are technically not valid JSON).

res.json(null);
res.json({ user: 'tobi' });
res.status(500).json({ error: 'message' });

res.jsonp([body])

Sends a JSON response with JSONP support. This method is identical to res.json(), except that it opts-in to JSONP callback support.

res.jsonp(null);
// => callback(null)

res.jsonp({ user: 'tobi' });
// => callback({ "user": "tobi" })

res.status(500).jsonp({ error: 'message' });
// => callback({ "error": "message" })

By default, the JSONP callback name is simply callback. Override this with the jsonp callback name setting.

The following are some examples of JSONP responses using the same code:

// ?callback=foo
res.jsonp({ user: 'tobi' });
// => foo({ "user": "tobi" })

app.set('jsonp callback name', 'cb');

// ?cb=foo
res.status(500).jsonp({ error: 'message' });
// => foo({ "error": "message" })

res.links(links)

Joins the links provided as properties of the parameter to populate the response’s Link HTTP header field.

For example, the following call:

res.links({
  next: 'http://api.example.com/users?page=2',
  last: 'http://api.example.com/users?page=5'
});

Yields the following results:

Link: <http://api.example.com/users?page=2>; rel="next",
      <http://api.example.com/users?page=5>; rel="last"

res.location(path)

Sets the response Location HTTP header to the specified path parameter.

res.location('/foo/bar');
res.location('http://example.com');
res.location('back');

A path value of “back” has a special meaning, it refers to the URL specified in the Referer header of the request. If the Referer header was not specified, it refers to “/”.

res.redirect([status,] path)

Redirects to the URL derived from the specified path, with specified status, a positive integer that corresponds to an HTTP status code . If not specified, status defaults to “302 “Found”.

res.redirect('/foo/bar');
res.redirect('http://example.com');
res.redirect(301, 'http://example.com');
res.redirect('../login');

Redirects can be a fully-qualified URL for redirecting to a different site:

res.redirect('http://google.com');

Redirects can be relative to the root of the host name. For example, if the application is on http://example.com/admin/post/new, the following would redirect to the URL http://example.com/admin:

res.redirect('/admin');

Redirects can be relative to the current URL. For example, from http://example.com/blog/admin/ (notice the trailing slash), the following would redirect to the URL http://example.com/blog/admin/post/new.

res.redirect('post/new');

Redirecting to post/new from http://example.com/blog/admin (no trailing slash), will redirect to http://example.com/blog/post/new.

If you found the above behavior confusing, think of path segments as directories (with trailing slashes) and files, it will start to make sense.

Path-relative redirects are also possible. If you were on http://example.com/admin/post/new, the following would redirect to http://example.com/admin/post:

res.redirect('..');

A back redirection redirects the request back to the referer, defaulting to / when the referer is missing.

res.redirect('back');

res.render(view [, locals] [, callback])

Renders a view and sends the rendered HTML string to the client. Optional parameters:

• locals, an object whose properties define local variables for the view.
• callback, a callback function. If provided, the method returns both the possible error and rendered string, but does not perform an automated response. When an error occurs, the method invokes next(err) internally.

The view argument is a string that is the file path of the view file to render. This can be an absolute path, or a path relative to the views setting. If the path does not contain a file extension, then the view engine setting determines the file extension. If the path does contain a file extension, then Express will load the module for the specified template engine (via require()) and render it using the loaded module’s __express function.

For more information, see Using template engines with Express.

NOTE: The view argument performs file system operations like reading a file from disk and evaluating Node.js modules, and as so for security reasons should not contain input from the end-user.

// send the rendered view to the client
res.render('index');

// if a callback is specified, the rendered HTML string has to be sent explicitly
res.render('index', function(err, html) {
  res.send(html);
});

// pass a local variable to the view
res.render('user', { name: 'Tobi' }, function(err, html) {
  // ...
});

res.send([body])

Sends the HTTP response.

The body parameter can be a Buffer object, a String, an object, or an Array. For example:

res.send(new Buffer('whoop'));
res.send({ some: 'json' });
res.send('<p>some html</p>');
res.status(404).send('Sorry, we cannot find that!');
res.status(500).send({ error: 'something blew up' });

This method performs many useful tasks for simple non-streaming responses: For example, it automatically assigns the Content-Length HTTP response header field (unless previously defined) and provides automatic HEAD and HTTP cache freshness support.

When the parameter is a Buffer object, the method sets the Content-Type response header field to “application/octet-stream”, unless previously defined as shown below:

res.set('Content-Type', 'text/html');
res.send(new Buffer('<p>some html</p>'));

When the parameter is a String, the method sets the Content-Type to “text/html”:

res.send('<p>some html</p>');

When the parameter is an Array or Object, Express responds with the JSON representation:

res.send({ user: 'tobi' });
res.send([1,2,3]);

res.sendFile(path [, options] [, fn])

Transfers the file at the given path. Sets the Content-Type response HTTP header field based on the filename’s extension. Unless the root option is set in the options object, path must be an absolute path to the file.

The following table provides details on the options parameter.

The method invokes the callback function fn(err) when the transfer is complete or when an error occurs. If the callback function is specified and an error occurs, the callback function must explicitly handle the response process either by ending the request-response cycle, or by passing control to the next route.

Here is an example of using res.sendFile with all its arguments.

app.get('/file/:name', function (req, res, next) {

  var options = {
    root: __dirname + '/public/',
    dotfiles: 'deny',
    headers: {
        'x-timestamp': Date.now(),
        'x-sent': true
    }
  };

  var fileName = req.params.name;
  res.sendFile(fileName, options, function (err) {
    if (err) {
      next(err);
    } else {
      console.log('Sent:', fileName);
    }
  });

});

The following example illustrates using res.sendFile to provide fine-grained support for serving files:

app.get('/user/:uid/photos/:file', function(req, res){
  var uid = req.params.uid
    , file = req.params.file;

  req.user.mayViewFilesFrom(uid, function(yes){
    if (yes) {
      res.sendFile('/uploads/' + uid + '/' + file);
    } else {
      res.status(403).send("Sorry! You can't see that.");
    }
  });
});

For more information, or if you have issues or concerns, see send.

res.sendStatus(statusCode)

Sets the response HTTP status code to statusCode and send its string representation as the response body.

res.sendStatus(200); // equivalent to res.status(200).send('OK')
res.sendStatus(403); // equivalent to res.status(403).send('Forbidden')
res.sendStatus(404); // equivalent to res.status(404).send('Not Found')
res.sendStatus(500); // equivalent to res.status(500).send('Internal Server Error')

If an unsupported status code is specified, the HTTP status is still set to statusCode and the string version of the code is sent as the response body.

res.sendStatus(2000); // equivalent to res.status(2000).send('2000')

More about HTTP Status Codes

res.set(field [, value])

Sets the response’s HTTP header field to value. To set multiple fields at once, pass an object as the parameter.

res.set('Content-Type', 'text/plain');

res.set({
  'Content-Type': 'text/plain',
  'Content-Length': '123',
  'ETag': '12345'
});

Aliased as res.header(field [, value]).

res.status(code)

Sets the HTTP status for the response. It is a chainable alias of Node’s response.statusCode.

res.status(403).end();
res.status(400).send('Bad Request');
res.status(404).sendFile('/absolute/path/to/404.png');

res.type(type)

Sets the Content-Type HTTP header to the MIME type as determined by mime.lookup() for the specified type. If type contains the “/” character, then it sets the Content-Type to type.

res.type('.html');              // => 'text/html'
res.type('html');               // => 'text/html'
res.type('json');               // => 'application/json'
res.type('application/json');   // => 'application/json'
res.type('png');                // => image/png:

res.vary(field)

Adds the field to the Vary response header, if it is not there already.

res.vary('User-Agent').render('docs');

Router

A router object is an isolated instance of middleware and routes. You can think of it as a “mini-application,” capable only of performing middleware and routing functions. Every Express application has a built-in app router.

A router behaves like middleware itself, so you can use it as an argument to app.use() or as the argument to another router’s use() method.

The top-level express object has a Router() method that creates a new router object.

Once you’ve created a router object, you can add middleware and HTTP method routes (such as get, put, post, and so on) to it just like an application. For example:

// invoked for any requests passed to this router
router.use(function(req, res, next) {
  // .. some logic here .. like any other middleware
  next();
});

// will handle any request that ends in /events
// depends on where the router is "use()'d"
router.get('/events', function(req, res, next) {
  // ..
});

You can then use a router for a particular root URL in this way separating your routes into files or even mini-apps.

// only requests to /calendar/* will be sent to our "router"
app.use('/calendar', router);

Methods

router.all(path, [callback, ...] callback)

This method is just like the router.METHOD() methods, except that it matches all HTTP methods (verbs).

This method is extremely useful for mapping “global” logic for specific path prefixes or arbitrary matches. For example, if you placed the following route at the top of all other route definitions, it would require that all routes from that point on would require authentication, and automatically load a user. Keep in mind that these callbacks do not have to act as end points; loadUser can perform a task, then call next() to continue matching subsequent routes.

router.all('*', requireAuthentication, loadUser);

Or the equivalent:

router.all('*', requireAuthentication)
router.all('*', loadUser);

Another example of this is white-listed “global” functionality. Here the example is much like before, but it only restricts paths prefixed with “/api”:

router.all('/api/*', requireAuthentication);

router.METHOD(path, [callback, ...] callback)

The router.METHOD() methods provide the routing functionality in Express, where METHOD is one of the HTTP methods, such as GET, PUT, POST, and so on, in lowercase. Thus, the actual methods are router.get(), router.post(), router.put(), and so on.

You can provide multiple callbacks, and all are treated equally, and behave just like middleware, except that these callbacks may invoke next('route') to bypass the remaining route callback(s). You can use this mechanism to perform pre-conditions on a route then pass control to subsequent routes when there is no reason to proceed with the route matched.

The following snippet illustrates the most simple route definition possible. Express translates the path strings to regular expressions, used internally to match incoming requests. Query strings are not considered when performing these matches, for example “GET /” would match the following route, as would “GET /?name=tobi”.

router.get('/', function(req, res){
  res.send('hello world');
});

You can also use regular expressions—useful if you have very specific constraints, for example the following would match “GET /commits/71dbb9c” as well as “GET /commits/71dbb9c..4c084f9”.

router.get(/^\/commits\/(\w+)(?:\.\.(\w+))?$/, function(req, res){
  var from = req.params[0];
  var to = req.params[1] || 'HEAD';
  res.send('commit range ' + from + '..' + to);
});

router.param(name, callback)

Adds callback triggers to route parameters, where name is the name of the parameter and callback is the callback function. Although name is technically optional, using this method without it is deprecated starting with Express v4.11.0 (see below).

The parameters of the callback function are:

• req, the request object.
• res, the response object.
• next, indicating the next middleware function.
• The value of the name parameter.
• The name of the parameter.

For example, when :user is present in a route path, you may map user loading logic to automatically provide req.user to the route, or perform validations on the parameter input.

router.param('user', function(req, res, next, id) {

  // try to get the user details from the User model and attach it to the request object
  User.find(id, function(err, user) {
    if (err) {
      next(err);
    } else if (user) {
      req.user = user;
      next();
    } else {
      next(new Error('failed to load user'));
    }
  });
});

Param callback functions are local to the router on which they are defined. They are not inherited by mounted apps or routers. Hence, param callbacks defined on router will be triggered only by route parameters defined on router routes.

A param callback will be called only once in a request-response cycle, even if the parameter is matched in multiple routes, as shown in the following examples.

router.param('id', function (req, res, next, id) {
  console.log('CALLED ONLY ONCE');
  next();
});

router.get('/user/:id', function (req, res, next) {
  console.log('although this matches');
  next();
});

router.get('/user/:id', function (req, res) {
  console.log('and this matches too');
  res.end();
});

On GET /user/42, the following is printed:

CALLED ONLY ONCE
although this matches
and this matches too

The behavior of the router.param(name, callback) method can be altered entirely by passing only a function to router.param(). This function is a custom implementation of how router.param(name, callback) should behave - it accepts two parameters and must return a middleware.

The first parameter of this function is the name of the URL parameter that should be captured, the second parameter can be any JavaScript object which might be used for returning the middleware implementation.

The middleware returned by the function decides the behavior of what happens when a URL parameter is captured.

In this example, the router.param(name, callback) signature is modified to router.param(name, accessId). Instead of accepting a name and a callback, router.param() will now accept a name and a number.

var express = require('express');
var app = express();
var router = express.Router();

// customizing the behavior of router.param()
router.param(function(param, option) {
  return function (req, res, next, val) {
    if (val == option) {
      next();
    }
    else {
      res.sendStatus(403);
    }
  }
});

// using the customized router.param()
router.param('id', 1337);

// route to trigger the capture
router.get('/user/:id', function (req, res) {
  res.send('OK');
});

app.use(router);

app.listen(3000, function () {
  console.log('Ready');
});

In this example, the router.param(name, callback) signature remains the same, but instead of a middleware callback, a custom data type checking function has been defined to validate the data type of the user id.

router.param(function(param, validator) {
  return function (req, res, next, val) {
    if (validator(val)) {
      next();
    }
    else {
      res.sendStatus(403);
    }
  }
});

router.param('id', function (candidate) {
  return !isNaN(parseFloat(candidate)) && isFinite(candidate);
});

router.route(path)

Returns an instance of a single route which you can then use to handle HTTP verbs with optional middleware. Use router.route() to avoid duplicate route naming and thus typing errors.

Building on the router.param() example above, the following code shows how to use router.route() to specify various HTTP method handlers.

var router = express.Router();

router.param('user_id', function(req, res, next, id) {
  // sample user, would actually fetch from DB, etc...
  req.user = {
    id: id,
    name: 'TJ'
  };
  next();
});

router.route('/users/:user_id')
.all(function(req, res, next) {
  // runs for all HTTP verbs first
  // think of it as route specific middleware!
  next();
})
.get(function(req, res, next) {
  res.json(req.user);
})
.put(function(req, res, next) {
  // just an example of maybe updating the user
  req.user.name = req.params.name;
  // save user ... etc
  res.json(req.user);
})
.post(function(req, res, next) {
  next(new Error('not implemented'));
})
.delete(function(req, res, next) {
  next(new Error('not implemented'));
});

This approach re-uses the single /users/:user_id path and adds handlers for various HTTP methods.

router.use([path], [function, ...] function)

Uses the specified middleware function or functions, with optional mount path path, that defaults to “/”.

This method is similar to app.use(). A simple example and use case is described below. See app.use() for more information.

Middleware is like a plumbing pipe: requests start at the first middleware function defined and work their way “down” the middleware stack processing for each path they match.

var express = require('express');
var app = express();
var router = express.Router();

// simple logger for this router's requests
// all requests to this router will first hit this middleware
router.use(function(req, res, next) {
  console.log('%s %s %s', req.method, req.url, req.path);
  next();
});

// this will only be invoked if the path starts with /bar from the mount point
router.use('/bar', function(req, res, next) {
  // ... maybe some additional /bar logging ...
  next();
});

// always invoked
router.use(function(req, res, next) {
  res.send('Hello World');
});

app.use('/foo', router);

app.listen(3000);

The “mount” path is stripped and is not visible to the middleware function. The main effect of this feature is that a mounted middleware function may operate without code changes regardless of its “prefix” pathname.

The order in which you define middleware with router.use() is very important. They are invoked sequentially, thus the order defines middleware precedence. For example, usually a logger is the very first middleware you would use, so that every request gets logged.

var logger = require('morgan');

router.use(logger());
router.use(express.static(__dirname + '/public'));
router.use(function(req, res){
  res.send('Hello');
});

Now suppose you wanted to ignore logging requests for static files, but to continue logging routes and middleware defined after logger(). You would simply move the call to express.static() to the top, before adding the logger middleware:

router.use(express.static(__dirname + '/public'));
router.use(logger());
router.use(function(req, res){
  res.send('Hello');
});

Another example is serving files from multiple directories, giving precedence to “./public” over the others:

app.use(express.static(__dirname + '/public'));
app.use(express.static(__dirname + '/files'));
app.use(express.static(__dirname + '/uploads'));

The router.use() method also supports named parameters so that your mount points for other routers can benefit from preloading using named parameters.

NOTE: Although these middleware functions are added via a particular router, when they run is defined by the path they are attached to (not the router). Therefore, middleware added via one router may run for other routers if its routes match. For example, this code shows two different routers mounted on the same path:

var authRouter = express.Router();
var openRouter = express.Router();

authRouter.use(require('./authenticate').basic(usersdb));

authRouter.get('/:user_id/edit', function(req, res, next) { 
  // ... Edit user UI ...  
});
openRouter.get('/', function(req, res, next) { 
  // ... List users ... 
})
openRouter.get('/:user_id', function(req, res, next) { 
  // ... View user ... 
})

app.use('/users', authRouter);
app.use('/users', openRouter);

Even though the authentication middleware was added via the authRouter it will run on the routes defined by the openRouter as well since both routers were mounted on /users. To avoid this behavior, use different paths for each router.