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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 “/”.

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.

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

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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.

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// this middleware will be executed for every request to the app
app.use(function (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.

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// 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.

Type Example
Path This will match paths starting with `/abcd`:
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app.use('/abcd', function (req, res, next) {
  next();
});
Path Pattern This will match paths starting with `/abcd` and `/abd`:
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app.use('/abc?d', function (req, res, next) {
  next();
});
This will match paths starting with `/abcd`, `/abbcd`, `/abbbbbcd`, and so on:
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app.use('/ab+cd', function (req, res, next) {
  next();
});
This will match paths starting with `/abcd`, `/abxcd`, `/abFOOcd`, `/abbArcd`, and so on:
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app.use('/ab\*cd', function (req, res, next) {
  next();
});
This will match paths starting with `/ad` and `/abcd`:
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app.use('/a(bc)?d', function (req, res, next) {
  next();
});
Regular Expression This will match paths starting with `/abc` and `/xyz`:
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app.use(/\/abc|\/xyz/, function (req, res, next) {
  next();
});
Array This will match paths starting with `/abcd`, `/xyza`, `/lmn`, and `/pqr`:
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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.

Usage Example
Single Middleware You can define and mount a middleware function locally.
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app.use(function (req, res, next) {
  next();
});
A router is valid middleware.
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var router = express.Router();
router.get('/', function (req, res, next) {
  next();
});
app.use(router);
An Express app is valid middleware.
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var subApp = express();
subApp.get('/', function (req, res, next) {
  next();
});
app.use(subApp);
Series of Middleware You can specify more than one middleware function at the same mount path.
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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);
Array Use an array to group middleware logically. If you pass an array of middleware as the first or only middleware parameters, then you must specify the mount path.
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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]);
Combination You can combine all the above ways of mounting middleware.
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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:

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// 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”:

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// 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:

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

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

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app.use(express.static(__dirname + '/public'));
app.use(express.static(__dirname + '/files'));
app.use(express.static(__dirname + '/uploads'));
doc_Express
doc_Express
2025-01-10 15:47:30
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