Express 5.x - API Reference

Caution

This is early beta documentation that may be incomplete and is still under development.

Caution

Express 5.0 requires Node.js 18 or higher.

express()

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

const express = require('express')
const 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.

Danger

As req.body’s shape is based on user-controlled input, all properties and values in this object are untrusted and should be validated before trusting. For example, req.body.foo.toString() may fail in multiple ways, for example foo may not be there or may not be a string, and toString may not be a function and instead a string or other user-input.

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

Property	Description	Type	Default
inflate	Enables or disables handling deflated (compressed) bodies; when disabled, deflated bodies are rejected.	Boolean	true
limit	Controls the maximum request body size. If this is a number, then the value specifies the number of bytes; if it is a string, the value is passed to the bytes library for parsing.	Mixed	"100kb"
reviver	The reviver option is passed directly to JSON.parse as the second argument. You can find more information on this argument in the MDN documentation about JSON.parse.	Function	null
strict	Enables or disables only accepting arrays and objects; when disabled will accept anything JSON.parse accepts.	Boolean	true
type	This is used to determine what media type the middleware will parse. This option can be a string, array of strings, or a function. If not a function, type option is passed directly to the type-is library and this can be an extension name (like json), a mime type (like application/json), or a mime type with a wildcard (like */* or */json). If a function, the type option is called as fn(req) and the request is parsed if it returns a truthy value.	Mixed	"application/json"
verify	This option, if supplied, is called as verify(req, res, buf, encoding), where buf is a Buffer of the raw request body and encoding is the encoding of the request. The parsing can be aborted by throwing an error.	Function	undefined

express.static(root, [options])

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

Note

For best results, use a reverse proxy cache to improve performance of serving static assets.

The root argument specifies the root directory from which to serve static assets. The function determines the file to serve 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.

Property	Description	Type	Default
dotfiles	Determines how dotfiles (files or directories that begin with a dot ”.”) are treated. See dotfiles below.	String	”ignore”
etag	Enable or disable etag generation NOTE: express.static always sends weak ETags.	Boolean	true
extensions	Sets file extension fallbacks: If a file is not found, search for files with the specified extensions and serve the first one found. Example: ['html', 'htm'].	Mixed	false
fallthrough	Let client errors fall-through as unhandled requests, otherwise forward a client error. See fallthrough below.	Boolean	true
immutable	Enable or disable the immutable directive in the Cache-Control response header. If enabled, the maxAge option should also be specified to enable caching. The immutable directive will prevent supported clients from making conditional requests during the life of the maxAge option to check if the file has changed.	Boolean	false
index	Sends the specified directory index file. Set to false to disable directory indexing.	Mixed	”index.html”
lastModified	Set the Last-Modified header to the last modified date of the file on the OS.	Boolean	true
maxAge	Set the max-age property of the Cache-Control header in milliseconds or a string in ms format.	Number	0
redirect	Redirect to trailing ”/” when the pathname is a directory.	Boolean	true
setHeaders	Function for setting HTTP headers to serve with the file. See setHeaders below.	Function

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().

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:

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

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

express.Router([options])

Creates a new router object.

const router = express.Router([options])

The optional options parameter specifies the behavior of the router.

Property	Description	Default	Availability
caseSensitive	Enable case sensitivity.	Disabled by default, treating “/Foo” and “/foo” as the same.
mergeParams	Preserve the req.params values from the parent router. If the parent and the child have conflicting param names, the child’s value take precedence.	false	4.5.0+
strict	Enable strict routing.	Disabled by default, “/foo” and “/foo/” are treated the same by 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).

Danger

As req.body’s shape is based on user-controlled input, all properties and values in this object are untrusted and should be validated before trusting. For example, req.body.foo.toString() may fail in multiple ways, for example foo may not be there or may not be a string, and toString may not be a function and instead a string or other user-input.

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

Property	Description	Type	Default
extended	This option allows to choose between parsing the URL-encoded data with the querystring library (when false) or the qs library (when true). The “extended” syntax allows for rich objects and arrays to be encoded into the URL-encoded format, allowing for a JSON-like experience with URL-encoded. For more information, please see the qs library.	Boolean	false
inflate	Enables or disables handling deflated (compressed) bodies; when disabled, deflated bodies are rejected.	Boolean	true
limit	Controls the maximum request body size. If this is a number, then the value specifies the number of bytes; if it is a string, the value is passed to the bytes library for parsing.	Mixed	"100kb"
parameterLimit	This option controls the maximum number of parameters that are allowed in the URL-encoded data. If a request contains more parameters than this value, an error will be raised.	Number	1000
type	This is used to determine what media type the middleware will parse. This option can be a string, array of strings, or a function. If not a function, type option is passed directly to the type-is library and this can be an extension name (like urlencoded), a mime type (like application/x-www-form-urlencoded), or a mime type with a wildcard (like */x-www-form-urlencoded). If a function, the type option is called as fn(req) and the request is parsed if it returns a truthy value.	Mixed	"application/x-www-form-urlencoded"
verify	This option, if supplied, is called as verify(req, res, buf, encoding), where buf is a Buffer of the raw request body and encoding is the encoding of the request. The parsing can be aborted by throwing an error.	Function	undefined

Application

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

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

app.get('/', (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.

Note

The Express application object can be referred from the request object and the response object as req.app, and res.app, respectively.

Properties

app.locals

The app.locals object has properties that are local variables within the application, and will be available in templates rendered with res.render.

console.dir(app.locals.title)
// => 'My App'

console.dir(app.locals.email)
// => 'me@myapp.com'

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 = 'me@myapp.com'

app.mountpath

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

Note

A sub-app is an instance of express that may be used for handling the request to a route.

const express = require('express')

const app = express() // the main app
const admin = express() // the sub app

admin.get('/', (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.

const admin = express()

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

const secret = express()
secret.get('/', (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

app.router

The application’s in-built instance of router. This is created lazily, on first access.

const express = require('express')
const app = express()
const router = app.router

router.get('/', (req, res) => {
  res.send('hello world')
})

app.listen(3000)

You can add middleware and HTTP method routes to the router just like an application.

For more information, see Router.

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.

Note

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.

For details, see Application settings.

const admin = express()

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

admin.get('/', (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

Argument	Description	Default
path	The path for which the middleware function is invoked; can be any of: A string representing a path. A path pattern. A regular expression pattern to match paths. An array of combinations of any of the above. For examples, see Path examples.	’/’ (root path)
callback	Callback functions; can be: A middleware function. A series of middleware functions (separated by commas). An array of middleware functions. A combination of all of the above. You can provide multiple callback functions that behave just like middleware, except that these callbacks can invoke next(‘route’) to bypass the remaining route callback(s). You can use this mechanism to impose pre-conditions on a route, then pass control to subsequent routes if there is no reason to proceed with the current route. When a callback function throws an error or returns a rejected promise, next(err) will be invoked automatically. Since router and app implement the middleware interface, you can use them as you would any other middleware function. For examples, see Middleware callback function examples.	None

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', (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

Argument	Description	Default
path	The path for which the middleware function is invoked; can be any of: A string representing a path. A path pattern. A regular expression pattern to match paths. An array of combinations of any of the above. For examples, see Path examples.	’/’ (root path)
callback	Callback functions; can be: A middleware function. A series of middleware functions (separated by commas). An array of middleware functions. A combination of all of the above. You can provide multiple callback functions that behave just like middleware, except that these callbacks can invoke next(‘route’) to bypass the remaining route callback(s). You can use this mechanism to impose pre-conditions on a route, then pass control to subsequent routes if there is no reason to proceed with the current route. When a callback function throws an error or returns a rejected promise, next(err) will be invoked automatically. Since router and app implement the middleware interface, you can use them as you would any other middleware function. For examples, see Middleware callback function examples.	None

Example

app.delete('/', (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.

const 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

Argument	Description	Default
path	The path for which the middleware function is invoked; can be any of: A string representing a path. A path pattern. A regular expression pattern to match paths. An array of combinations of any of the above. For examples, see Path examples.	’/’ (root path)
callback	Callback functions; can be: A middleware function. A series of middleware functions (separated by commas). An array of middleware functions. A combination of all of the above. You can provide multiple callback functions that behave just like middleware, except that these callbacks can invoke next(‘route’) to bypass the remaining route callback(s). You can use this mechanism to impose pre-conditions on a route, then pass control to subsequent routes if there is no reason to proceed with the current route. When a callback function throws an error or returns a rejected promise, next(err) will be invoked automatically. Since router and app implement the middleware interface, you can use them as you would any other middleware function. For examples, see Middleware callback function examples.	None

For more information, see the routing guide.

Example

app.get('/', (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().

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

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().

If port is omitted or is 0, the operating system will assign an arbitrary unused port, which is useful for cases like automated tasks (tests, etc.).

const express = require('express')
const 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):

const express = require('express')
const https = require('https')
const http = require('http')
const 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 () {
  const server = http.createServer(this)
  return server.listen.apply(server, arguments)
}

Note

All the forms of Node’s http.Server.listen() method are in fact actually supported.

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

Argument	Description	Default
path	The path for which the middleware function is invoked; can be any of: A string representing a path. A path pattern. A regular expression pattern to match paths. An array of combinations of any of the above. For examples, see Path examples.	’/’ (root path)
callback	Callback functions; can be: A middleware function. A series of middleware functions (separated by commas). An array of middleware functions. A combination of all of the above. You can provide multiple callback functions that behave just like middleware, except that these callbacks can invoke next(‘route’) to bypass the remaining route callback(s). You can use this mechanism to impose pre-conditions on a route, then pass control to subsequent routes if there is no reason to proceed with the current route. When a callback function throws an error or returns a rejected promise, next(err) will be invoked automatically. Since router and app implement the middleware interface, you can use them as you would any other middleware function. For examples, see Middleware callback function examples.	None

Routing methods

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

checkout copy delete get head lock merge mkactivity

mkcol move m-search notify options patch post

purge put report search subscribe trace unlock unsubscribe

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 ....

Note

The app.get() function is automatically called for the HTTP HEAD method in addition to the GET method if app.head() was not called for the path before app.get().

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', (req, res, next, id) => {
  // try to get the user details from the User model and attach it to the request object
  User.find(id, (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, nor are they triggered for route parameters inherited from parent 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', (req, res, next, id) => {
  console.log('CALLED ONLY ONCE')
  next()
})

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

app.get('/user/:id', (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'], (req, res, next, value) => {
  console.log('CALLED ONLY ONCE with', value)
  next()
})

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

app.get('/user/:id/:page', (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

app.path()

Returns the canonical path of the app, 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'

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

Argument	Description	Default
path	The path for which the middleware function is invoked; can be any of: A string representing a path. A path pattern. A regular expression pattern to match paths. An array of combinations of any of the above. For examples, see Path examples.	’/’ (root path)
callback	Callback functions; can be: A middleware function. A series of middleware functions (separated by commas). An array of middleware functions. A combination of all of the above. You can provide multiple callback functions that behave just like middleware, except that these callbacks can invoke next(‘route’) to bypass the remaining route callback(s). You can use this mechanism to impose pre-conditions on a route, then pass control to subsequent routes if there is no reason to proceed with the current route. When a callback function throws an error or returns a rejected promise, next(err) will be invoked automatically. Since router and app implement the middleware interface, you can use them as you would any other middleware function. For examples, see Middleware callback function examples.	None

Example

app.post('/', (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

Argument	Description	Default
path	The path for which the middleware function is invoked; can be any of: A string representing a path. A path pattern. A regular expression pattern to match paths. An array of combinations of any of the above. For examples, see Path examples.	’/’ (root path)
callback	Callback functions; can be: A middleware function. A series of middleware functions (separated by commas). An array of middleware functions. A combination of all of the above. You can provide multiple callback functions that behave just like middleware, except that these callbacks can invoke next(‘route’) to bypass the remaining route callback(s). You can use this mechanism to impose pre-conditions on a route, then pass control to subsequent routes if there is no reason to proceed with the current route. When a callback function throws an error or returns a rejected promise, next(err) will be invoked automatically. Since router and app implement the middleware interface, you can use them as you would any other middleware function. For examples, see Middleware callback function examples.	None

Example

app.put('/', (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.

Note

Think of app.render() as a utility function for generating rendered view strings. Internally res.render() uses app.render() to render views.

Caution

The local variable cache is reserved for enabling view cache. Set it to true, if you want to cache view during development; view caching is enabled in production by default.

app.render('email', (err, html) => {
  // ...
})

app.render('email', { name: 'Tobi' }, (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).

const app = express()

app.route('/events')
  .all((req, res, next) => {
    // runs for all HTTP verbs first
    // think of it as route specific middleware!
  })
  .get((req, res, next) => {
    res.json({})
  })
  .post((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”).

Property	Type	Description	Default
case sensitive routing	Boolean	Enable case sensitivity. When enabled, “/Foo” and “/foo” are different routes. When disabled, “/Foo” and “/foo” are treated the same. Note Sub-apps will inherit the value of this setting.	N/A (undefined)
env	String	Environment mode. Be sure to set to “production” in a production environment; see Production best practices: performance and reliability.	process.env.NODE_ENV (NODE_ENV environment variable) or “development” if NODE_ENV is not set.
etag	Varied	Set the ETag response header. For possible values, see the etag options table. More about the HTTP ETag header.	weak
jsonp callback name	String	Specifies the default JSONP callback name.	”callback”
json escape	Boolean	Enable escaping JSON responses from the res.json, res.jsonp, and res.send APIs. This will escape the characters <, >, and & as Unicode escape sequences in JSON. The purpose of this is to assist with mitigating certain types of persistent XSS attacks when clients sniff responses for HTML. Note Sub-apps will inherit the value of this setting.	N/A (undefined)
json replacer	Varied	The ‘replacer’ argument used by JSON.stringify. Note Sub-apps will inherit the value of this setting.	N/A (undefined)
json spaces	Varied	The ‘space’ argument used by JSON.stringify. This is typically set to the number of spaces to use to indent prettified JSON. Note Sub-apps will inherit the value of this setting.	N/A (undefined)
query parser	Varied	Disable query parsing by setting the value to false, or set the query parser to use either “simple” or “extended” or a custom query string parsing function. The simple query parser is based on Node’s native query parser, querystring. The extended query parser is based on qs. A custom query string parsing function will receive the complete query string, and must return an object of query keys and their values.	”simple”
strict routing	Boolean	Enable strict routing. When enabled, the router treats “/foo” and “/foo/” as different. Otherwise, the router treats “/foo” and “/foo/” as the same. Note Sub-apps will inherit the value of this setting.	N/A (undefined)
subdomain offset	Number	The number of dot-separated parts of the host to remove to access subdomain.	2
trust proxy	Varied	Indicates the app is behind a front-facing proxy, and to use the X-Forwarded-* headers to determine the connection and the IP address of the client. NOTE: X-Forwarded-* headers are easily spoofed and the detected IP addresses are unreliable. When enabled, Express attempts to determine the IP address of the client connected through the front-facing proxy, or series of proxies. The req.ips property, then contains an array of IP addresses the client is connected through. To enable it, use the values described in the trust proxy options table. The trust proxy setting is implemented using the proxy-addr package. For more information, see its documentation. Note Sub-apps will inherit the value of this setting, even though it has a default value.	false (disabled)
views	String or Array	A directory or an array of directories for the application’s views. If an array, the views are looked up in the order they occur in the array.	process.cwd() + '/views'
view cache	Boolean	Enables view template compilation caching. Note Sub-apps will not inherit the value of this setting in production (when NODE_ENV is “production”).	true in production, otherwise undefined.
view engine	String	The default engine extension to use when omitted. Note Sub-apps will inherit the value of this setting.	N/A (undefined)
x-powered-by	Boolean	Enables the “X-Powered-By: Express” HTTP header.	true

Options for trust proxy setting

Read Express behind proxies for more information.

Type	Value
Boolean	If true, the client’s IP address is understood as the left-most entry in the X-Forwarded-* header. If false, the app is understood as directly facing the Internet and the client’s IP address is derived from req.connection.remoteAddress. This is the default setting.
String String containing comma-separated values Array of strings	An IP address, subnet, or an array of IP addresses, and subnets to trust. Pre-configured subnet names are: loopback - 127.0.0.1/8, ::1/128 linklocal - 169.254.0.0/16, fe80::/10 uniquelocal - 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16, fc00::/7 Set IP addresses in any of the following ways: Specify a single subnet: app.set('trust proxy', 'loopback') Specify a subnet and an address: app.set('trust proxy', 'loopback, 123.123.123.123') Specify multiple subnets as CSV: app.set('trust proxy', 'loopback, linklocal, uniquelocal') Specify multiple subnets as an array: app.set('trust proxy', ['loopback', 'linklocal', 'uniquelocal']) When specified, the IP addresses or the subnets are excluded from the address determination process, and the untrusted IP address nearest to the application server is determined as the client’s IP address.
Number	Trust the nth hop from the front-facing proxy server as the client.
Function	Custom trust implementation. Use this only if you know what you are doing. app.set('trust proxy', (ip) => { if (ip === '127.0.0.1' || ip === '123.123.123.123') return true // trusted IPs else return false })

Options for etag setting

Note

These settings apply only to dynamic files, not static files. The express.static middleware ignores these settings.

The ETag functionality is implemented using the etag package. For more information, see its documentation.

Type	Value
Boolean	true enables weak ETag. This is the default setting. false disables ETag altogether.
String	If “strong”, enables strong ETag. If “weak”, enables weak ETag.
Function	Custom ETag function implementation. Use this only if you know what you are doing. app.set('etag', (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

Argument	Description	Default
path	The path for which the middleware function is invoked; can be any of: A string representing a path. A path pattern. A regular expression pattern to match paths. An array of combinations of any of the above. For examples, see Path examples.	’/’ (root path)
callback	Callback functions; can be: A middleware function. A series of middleware functions (separated by commas). An array of middleware functions. A combination of all of the above. You can provide multiple callback functions that behave just like middleware, except that these callbacks can invoke next(‘route’) to bypass the remaining route callback(s). You can use this mechanism to impose pre-conditions on a route, then pass control to subsequent routes if there is no reason to proceed with the current route. When a callback function throws an error or returns a rejected promise, next(err) will be invoked automatically. Since router and app implement the middleware interface, you can use them as you would any other middleware function. For examples, see Middleware callback function examples.	None

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((req, res, next) => {
  console.log('Time: %d', Date.now())
  next()
})

Note

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.

For details, see Application settings.

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((req, res, next) => {
  res.send('Hello World')
})

// requests will never reach this route
app.get('/', (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((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.

Type	Example
Path	This will match paths starting with /abcd: app.use('/abcd', (req, res, next) => { next() })
Path Pattern	This will match paths starting with /abcd and /abd: app.use('/ab(c?)d', (req, res, next) => { next() })
Regular Expression	This will match paths starting with /abc and /xyz: app.use(/\/abc|\/xyz/, (req, res, next) => { next() })
Array	This will match paths starting with /abcd, /xyza, /lmn, and /pqr: app.use(['/abcd', '/xyza', /\/lmn|\/pqr/], (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().

Usage	Example
Single Middleware	You can define and mount a middleware function locally. app.use((req, res, next) => { next() }) A router is valid middleware. const router = express.Router() router.get('/', (req, res, next) => { next() }) app.use(router) An Express app is valid middleware. const subApp = express() subApp.get('/', (req, res, next) => { next() }) app.use(subApp)
Series of Middleware	You can specify more than one middleware function at the same mount path. const r1 = express.Router() r1.get('/', (req, res, next) => { next() }) const r2 = express.Router() r2.get('/', (req, res, next) => { next() }) app.use(r1, r2)
Array	Use an array to group middleware logically. const r1 = express.Router() r1.get('/', (req, res, next) => { next() }) const r2 = express.Router() r2.get('/', (req, res, next) => { next() }) app.use([r1, r2])
Combination	You can combine all the above ways of mounting middleware. function mw1 (req, res, next) { next() } function mw2 (req, res, next) { next() } const r1 = express.Router() r1.get('/', (req, res, next) => { next() }) const r2 = express.Router() r2.get('/', (req, res, next) => { next() }) const subApp = express() subApp.get('/', (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(path.join(__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(path.join(__dirname, 'public')))

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

app.use(express.static(path.join(__dirname, 'public')))
app.use(logger())

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

app.use(express.static(path.join(__dirname, 'public')))
app.use(express.static(path.join(__dirname, 'files')))
app.use(express.static(path.join(__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', (req, res) => {
  res.send(`user ${req.params.id}`)
})

But you could just as well have:

app.get('/user/:id', (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

Caution

In Express 4, req.files is no longer available on the req object by default. To access uploaded files on the req.files object, use multipart-handling middleware like [busboy](https://www.npmjs. com/package/busboy), multer, formidable, multiparty, connect-multiparty, or pez.

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 = (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:

const greet = express.Router()

greet.get('/jp', (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.

Danger

As req.body’s shape is based on user-controlled input, all properties and values in this object are untrusted and should be validated before trusting. For example, req.body.foo.toString() may fail in multiple ways, for example foo may not be there or may not be a string, and toString may not be a function and instead a string or other user-input.

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

const app = require('express')()
const bodyParser = require('body-parser')
const multer = require('multer') // v1.0.5
const 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(), (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
console.dir(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

When the response is still “fresh” in the client’s cache true is returned, otherwise false is returned to indicate that the client cache is now stale and the full response should be sent.

When a client sends the Cache-Control: no-cache request header to indicate an end-to-end reload request, this module will return false to make handling these requests transparent.

Further details for how cache validation works can be found in the HTTP/1.1 Caching Specification.

console.dir(req.fresh)
// => true

req.host

Contains the host derived from the Host HTTP header.

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

If there is more than one X-Forwarded-Host header in the request, the value of the first header is used. This includes a single header with comma-separated values, in which the first value is used.

// Host: "example.com:3000"
console.dir(req.host)
// => 'example.com:3000'

// Host: "[::1]:3000"
console.dir(req.host)
// => '[::1]:3000'

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 get the value from the X-Forwarded-Host header field. This header can be set by the client or by the proxy.

If there is more than one X-Forwarded-Host header in the request, the value of the first header is used. This includes a single header with comma-separated values, in which the first value is used.

Note

Prior to Express v4.17.0, the X-Forwarded-Host could not contain multiple values or be present more than once.

// Host: "example.com:3000"
console.dir(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.

console.dir(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

Caution

req.url is not a native Express property, it is inherited from Node’s http module.

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
console.dir(req.originalUrl)
// => "/search?q=something"

req.originalUrl is available both in middleware and router objects, and is a combination of req.baseUrl and req.url. Consider following example:

// GET 'http://www.example.com/admin/new?sort=desc'
app.use('/admin', (req, res, next) => {
  console.dir(req.originalUrl) // '/admin/new?sort=desc'
  console.dir(req.baseUrl) // '/admin'
  console.dir(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
console.dir(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
console.dir(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.

Note

Express automatically decodes the values in req.params (using decodeURIComponent).

req.path

Contains the path part of the request URL.

// example.com/users?sort=desc
console.dir(req.path)
// => "/users"

Note

When called from a middleware, the mount point is not included in req.path. See app.use() for more details.

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.

console.dir(req.protocol)
// => "http"

req.query

This property is an object containing a property for each query string parameter in the route. When query parser is set to disabled, it is an empty object {}, otherwise it is the result of the configured query parser.

Danger

As req.query’s shape is based on user-controlled input, all properties and values in this object are untrusted and should be validated before trusting. For example, req.query.foo.toString() may fail in multiple ways, for example foo may not be there or may not be a string, and toString may not be a function and instead a string or other user-input.

The value of this property can be configured with the query parser application setting to work how your application needs it. A very popular query string parser is the qs module, and this is used by default. The qs module is very configurable with many settings, and it may be desirable to use different settings than the default to populate req.query:

const qs = require('qs')
app.set('query parser',
  (str) => qs.parse(str, { /* custom options */ }))

Check out the query parser application setting documentation for other customization options.

req.res

This property holds a reference to the response object that relates to this request object.

req.route

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

app.get('/user/:id?', (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 the following:

req.protocol === 'https'

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
console.dir(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.

console.dir(req.stale)
// => true

req.subdomains

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

// Host: "tobi.ferrets.example.com"
console.dir(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.

console.dir(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')
// => false

// 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. If the request has no body, returns null. 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.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.

| Property | Type | Description | |-------------|-------------------------------------------------------------------------| | combine | Boolean | Specify if overlapping & adjacent ranges should be combined, defaults to false. When true, ranges will be combined and returned as if they were specified that way in the header.

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
const range = req.range(1000)

// the type of the range
if (range.type === 'bytes') {
  // the ranges
  range.forEach((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', (req, res) => {
  res.send(`user ${req.params.id}`)
})

But you could just as well have:

app.get('/user/:id', (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('/', (req, res) => {
  console.log(res.headersSent) // false
  res.send('OK')
  console.log(res.headersSent) // true
})

res.locals

Use this property to set variables accessible in templates rendered with res.render. The variables set on res.locals are available within a single request-response cycle, and will not be shared between requests.

In order to keep local variables for use in template rendering between requests, use app.locals instead.

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

app.use((req, res, next) => {
  // Make `user` and `authenticated` available in templates
  res.locals.user = req.user
  res.locals.authenticated = !req.user.anonymous
  next()
})

res.req

This property holds a reference to the request object that relates to this response object.

Methods

res.append(field [, value])

Note

res.append() is supported by Express v4.11.0+

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.

| Property | Type | Description | |-------------|-------------------------------------------------------------------------| | domain | String | Domain name for the cookie. Defaults to the domain name of the app. | encode | Function | A synchronous function used for cookie value encoding. Defaults to encodeURIComponent. | expires | Date | Expiry date of the cookie in GMT. If not specified or set to 0, creates a session cookie. | httpOnly | Boolean | Flags the cookie to be accessible only by the web server. | maxAge | Number | Convenient option for setting the expiry time relative to the current time in milliseconds. | path | String | Path for the cookie. Defaults to ”/“. | secure | Boolean | Marks the cookie to be used with HTTPS only. | signed | Boolean | Indicates if the cookie should be signed. | sameSite | Boolean or String | Value of the “SameSite” Set-Cookie attribute. More information at https://tools.ietf.org/html/draft-ietf-httpbis-cookie-same-site-00#section-4.1.1.

Caution

All res.cookie() does is set the HTTP Set-Cookie header with the options provided. Any option not specified defaults to the value stated in RFC 6265.

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.signedCookies object.

res.clearCookie(name [, options])

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

Caution

Web browsers and other compliant clients will only clear the cookie if the given options is identical to those given to res.cookie(), excluding expires and maxAge.

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

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

Note

The optional options argument is supported by Express v4.16.0 onwards.

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 derived from the path argument, but can be overridden with the filename parameter. If path is relative, then it will be based on the current working directory of the process.

The following table provides details on the options parameter.

Note

The optional options argument is supported by Express v4.16.0 onwards.

Property	Description	Default	Availability
maxAge	Sets the max-age property of the Cache-Control header in milliseconds or a string in ms format	0	4.16+
lastModified	Sets the Last-Modified header to the last modified date of the file on the OS. Set false to disable it.	Enabled	4.16+
headers	Object containing HTTP headers to serve with the file. The header Content-Disposition will be overridden by the filename argument.		4.16+
dotfiles	Option for serving dotfiles. Possible values are “allow”, “deny”, “ignore”.	“ignore”	4.16+
acceptRanges	Enable or disable accepting ranged requests.	true	4.16+
cacheControl	Enable or disable setting Cache-Control response header.	true	4.16+
immutable	Enable or disable the immutable directive in the Cache-Control response header. If enabled, the maxAge option should also be specified to enable caching. The immutable directive will prevent supported clients from making conditional requests during the life of the maxAge option to check if the file has changed.	false	4.16+

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.

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

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

res.download('/report-12345.pdf', 'report.pdf', (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' () {
    res.send('hey')
  },

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

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

  default () {
    // 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 () {
    res.send('hey')
  },

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

  json () {
    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, number, or null, and you can also use it to convert other values to 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 ”/“.

See also Security best practices: Prevent open redirect vulnerabilities.

Danger

After encoding the URL, if not encoded already, Express passes the specified URL to the browser in the Location header, without any validation.

Browsers take the responsibility of deriving the intended URL from the current URL or the referring URL, and the URL specified in the Location header; and redirect the user accordingly.

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')

See also Security best practices: Prevent open redirect vulnerabilities.

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.

Caution

The local variable cache enables view caching. Set it to true, to cache the view during development; view caching is enabled in production by default.

// 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', (err, html) => {
  res.send(html)
})

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

res.send([body])

Sends the HTTP response.

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

res.send(Buffer.from('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 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(Buffer.from('<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])

Note

res.sendFile() is supported by Express v4.8.0 onwards.

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.

Danger

This API provides access to data on the running file system. Ensure that either (a) the way in which the path argument was constructed into an absolute path is secure if it contains user input or (b) set the root option to the absolute path of a directory to contain access within.

When the root option is provided, the path argument is allowed to be a relative path, including containing ... Express will validate that the relative path provided as path will resolve within the given root option.

The following table provides details on the options parameter.

Property	Description	Default	Availability
maxAge	Sets the max-age property of the Cache-Control header in milliseconds or a string in ms format	0
root	Root directory for relative filenames.
lastModified	Sets the Last-Modified header to the last modified date of the file on the OS. Set false to disable it.	Enabled	4.9.0+
headers	Object containing HTTP headers to serve with the file.
dotfiles	Option for serving dotfiles. Possible values are “allow”, “deny”, “ignore”.	“ignore”
acceptRanges	Enable or disable accepting ranged requests.	true	4.14+
cacheControl	Enable or disable setting Cache-Control response header.	true	4.14+
immutable	Enable or disable the immutable directive in the Cache-Control response header. If enabled, the maxAge option should also be specified to enable caching. The immutable directive will prevent supported clients from making conditional requests during the life of the maxAge option to check if the file has changed.	false	4.16+

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', (req, res, next) => {
  const options = {
    root: path.join(__dirname, 'public'),
    dotfiles: 'deny',
    headers: {
      'x-timestamp': Date.now(),
      'x-sent': true
    }
  }

  const fileName = req.params.name
  res.sendFile(fileName, options, (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', (req, res) => {
  const uid = req.params.uid
  const file = req.params.file

  req.user.mayViewFilesFrom(uid, (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 sends the registered status message as the text response body. If an unknown status code is specified, the response body will just be the code number.

res.sendStatus(404)

Caution

Some versions of Node.js will throw when res.statusCode is set to an invalid HTTP status code (outside of the range 100 to 599). Consult the HTTP server documentation for the Node.js version being used.

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 the specified type. If type contains the ”/” character, then it sets the Content-Type to the exact value of type, otherwise it is assumed to be a file extension and the MIME type is looked up in a mapping using the express.static.mime.lookup() method.

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((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', (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.

Note

The router.get() function is automatically called for the HTTP HEAD method in addition to the GET method if router.head() was not called for the path before router.get().

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('/', (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+))?$/, (req, res) => {
  const from = req.params[0]
  const to = req.params[1] || 'HEAD'
  res.send(`commit range ${from}..${to}`)
})

You can use next primitive to implement a flow control between different middleware functions, based on a specific program state. Invoking next with the string 'router' will cause all the remaining route callbacks on that router to be bypassed.

The following example illustrates next('router') usage.

function fn (req, res, next) {
  console.log('I come here')
  next('router')
}
router.get('/foo', fn, (req, res, next) => {
  console.log('I dont come here')
})
router.get('/foo', (req, res, next) => {
  console.log('I dont come here')
})
app.get('/foo', (req, res) => {
  console.log(' I come here too')
  res.end('good')
})

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.

Note

Unlike app.param(), router.param() does not accept an array of route parameters.

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', (req, res, next, id) => {
  // try to get the user details from the User model and attach it to the request object
  User.find(id, (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, nor are they triggered for route parameters inherited from parent 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', (req, res, next, id) => {
  console.log('CALLED ONLY ONCE')
  next()
})

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

router.get('/user/:id', (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

Danger

The following section describes router.param(callback), which is deprecated as of v4.11.0.

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.

const express = require('express')
const app = express()
const router = express.Router()

// customizing the behavior of router.param()
router.param((param, option) => {
  return (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', (req, res) => {
  res.send('OK')
})

app.use(router)

app.listen(3000, () => {
  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((param, validator) => {
  return (req, res, next, val) => {
    if (validator(val)) {
      next()
    } else {
      res.sendStatus(403)
    }
  }
})

router.param('id', (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.

const router = express.Router()

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

router.route('/users/:user_id')
  .all((req, res, next) => {
  // runs for all HTTP verbs first
  // think of it as route specific middleware!
    next()
  })
  .get((req, res, next) => {
    res.json(req.user)
  })
  .put((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((req, res, next) => {
    next(new Error('not implemented'))
  })
  .delete((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.

Note

When you use router.route(), middleware ordering is based on when the route is created, not when method handlers are added to the route. For this purpose, you can consider method handlers to belong to the route to which they were added.

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.

const express = require('express')
const app = express()
const router = express.Router()

// simple logger for this router's requests
// all requests to this router will first hit this middleware
router.use((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', (req, res, next) => {
  // ... maybe some additional /bar logging ...
  next()
})

// always invoked
router.use((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.

const logger = require('morgan')

router.use(logger())
router.use(express.static(path.join(__dirname, 'public')))
router.use((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(path.join(__dirname, 'public')))
router.use(logger())
router.use((req, res) => {
  res.send('Hello')
})

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

app.use(express.static(path.join(__dirname, 'public')))
app.use(express.static(path.join(__dirname, 'files')))
app.use(express.static(path.join(__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:

const authRouter = express.Router()
const openRouter = express.Router()

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

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