app.use()

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

Mounts the specified middleware function or functions at the specified path. If path is not specified, it defaults to “/”.

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

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

Mounting a middleware function at a path will cause the middleware function to be executed whenever the base of the requested path matches the path.

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

// this middleware will be executed for every request to the app
app.use(function (req, res, next) {
  console.log('Time: %d', Date.now());
  next();
});

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

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

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

path can be a string representing a path, a path pattern, a regular expression to match paths, or an array of combinations thereof.

The following table provides some simple examples of mounting middleware.

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

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

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

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

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

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

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

function can be a middleware function, a series of middleware functions, an array of middleware functions, or a combination of all of them. Since router and app implement the middleware interface, you can use them as you would any other middleware function.

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

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

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

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

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

app.use(r1, r2);

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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