`app.param()`

Registers a callback that runs whenever a given route parameter is present.

Since Express 4 Spec ↗

Syntax

app.param(name, callback)

Parameters

Name	Type	Required	Description
`name`	`string`	Yes	The route parameter name to hook, e.g. `'id'`.
`callback`	`function`	Yes	`(req, res, next, value)` invoked once per request when the param appears.

Returns

Application — The app instance, for chaining.

Examples

app.param('userId', async (req, res, next, id) => {
  const user = await findUser(id);
  if (!user) return res.sendStatus(404);
  req.user = user;
  next();
});

app.get('/users/:userId', (req, res) => res.json(req.user));

Output

$ curl localhost:3000/users/1
{"id":"1","name":"Ada"}

app.param('id', (req, res, next, id) => {
  if (!/^\d+$/.test(id)) return res.status(400).send('bad id');
  next();
});

Output

$ curl localhost:3000/users/abc
bad id

Notes

Runs before any route handler that uses the parameter, so it is ideal for centralizing resource loading and validation. It fires once per request even if multiple routes share the param.