app.use([path], function)

Use the given middleware function, with optional mount path, defaulting to ”/”.

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

// simple logger
app.use(function (req, res, next) {
  console.log('%s %s', req.method, req.url);
  next();
});

// respond
app.use(function (req, res, next) {
  res.send('Hello World');
});

app.listen(3000);

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

Here’s a concrete example, take the typical use-case of serving files in ./public using the express.static() middleware:

// GET /javascripts/jquery.js
// GET /style.css
// GET /favicon.ico
app.use(express.static(path.join(__dirname, 'public')));

Say for example you wanted to prefix all static files with “/static”, you could use the “mounting” feature to support this. Mounted middleware functions are not invoked unless the req.url contains this prefix, at which point it is stripped when the function is invoked. This affects this function only, subsequent middleware will see req.url with “/static” included unless they are mounted as well.

// GET /static/javascripts/jquery.js
// GET /static/style.css
// GET /static/favicon.ico
app.use('/static', express.static(path.join(__dirname, 'public')));

The order of which middleware are “defined” using app.use() is very important, they are invoked sequentially, thus this defines middleware precedence. For example usually express.logger() is the very first middleware you would use, logging every request:

app.use(express.logger());
app.use(express.static(path.join(__dirname, 'public')));
app.use(function (req, res) {
  res.send('Hello');
});

Now suppose you wanted to ignore logging requests for static files, but to continue logging routes and middleware defined after logger(), you would simply move static() above:

app.use(express.static(path.join(__dirname, 'public')));
app.use(express.logger());
app.use(function (req, res) {
  res.send('Hello');
});

Another concrete example would be 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')));