What Does `app.use(express.static())` Do in Express?

In Express, app.use(express.static()) adds a middleware for serving static files to your Express app.

For example, suppose you have the below public directory in your project:

$ ls -l public/
total 48
-rw-r--r--  1 ubuntu  ubuntu   1666 Mar 12 14:17 home.css
-rw-r--r--@ 1 ubuntu  ubuntu  17092 Mar 12 14:17 logo.png
$ 

You can use the express.static middleware to make it possible to access files from this folder via HTTP.

const express = require('express');

const app = express();
// The first parameter to `express.static()` is the directory to serve.
app.use(express.static('./public'));

app.listen(3000);

With the above script, you can open http://localhost:3000/home.css in your browser and see the CSS file.

Order Matters!

Keep in mind the app.use() function executes middleware in order. The express.static() middleware returns an HTTP 404 if it can't find a file, so that means you should typically call app.use(express.static()) after the rest of your app. Otherwise you'll end up with an HTTP 404 error:

const express = require('express');

const app = express();
app.use(express.static('./public'));

// Don't do this! If you send a request to `GET /test`, you'll get an
// HTTP 404 because this route handler is after `express.static()`!
app.get('/test', function requestHandler(req, res) {
  res.send('ok');
});

app.listen(3000);

Make sure you put express.static() last:

const express = require('express');

const app = express();

// Works! Express will handle requests to `/test`, and defer to
// the `static` middleware for requests for other URLs.
app.get('/test', function requestHandler(req, res) {
  res.send('ok');
});

app.use(express.static('./public'));

app.listen(3000);

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• The `app.use()` Function in Express
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