`router.param()`

Registers a parameter callback scoped to a router.

Since Express 4 Spec ↗

Syntax

router.param(name, callback)

Parameters

Name	Type	Required	Description
`name`	`string`	Yes	The route parameter name to hook.
`callback`	`function`	Yes	`(req, res, next, value)` run once when the param is matched.

Returns

Router — The router, for chaining.

Examples

const router = express.Router();

router.param('id', async (req, res, next, id) => {
  const book = await findBook(id);
  if (!book) return res.sendStatus(404);
  req.book = book;
  next();
});

router.get('/books/:id', (req, res) => res.json(req.book));
app.use('/api', router);

Output

$ curl localhost:3000/api/books/1
{"id":"1","title":"SICP"}

router.param('slug', (req, res, next, slug) => {
  req.slug = slug.toLowerCase();
  next();
});

Notes

Same behavior as `app.param` but limited to the router, so resource loading lives next to the routes that use it. Runs before the route's handlers and only once per request per param.