An Express router provides a subset of Express methods. To create an instance of one, we invoke the .Router() method on the top-level Express import.

To use a router, we mount it at a certain path using app.use() and pass in the router as the second argument. This router will now be used for all paths that begin with that path segment. To create a router to handle all requests beginning with /monsters, the code would look like this:

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

const monsters = {
  '1': {
    name: 'godzilla',
    age: 250000000
  },
  '2': {
    name: 'manticore',
    age: 21
  }
}

const monstersRouter = express.Router();

app.use('/monsters', monstersRouter);

monstersRouter.get('/:id', (req, res, next) => {
  const monster = monsters[req.params.id];
  if (monster) {
    res.send(monster);
  } else {
    res.status(404).send();
  }
});

Inside the monstersRouter, all matching routes are assumed to have /monsters prepended, as it is mounted at that path. monstersRouter.get('/:id') matches the full path /monsters/:id.

When a GET /monsters/1 request arrives, Express matches /monsters in app.use() because the beginning of the path ('/monsters') matches. Express’ route-matching algorithm enters the monstersRouter‘s routes to search for full path matches. Since monstersRouter.get('/:id) is mounted at /monsters, the two paths together match the entire request path (/monsters/1), so the route matches and the callback is invoked. The 'godzilla' monster is fetched from the monsters object and sent back.