Whenever the Express app receives a request to a route that matches a path containing the :userId param in it, the app will first execute the userByID controller function before propagating to the next function specific to the request that came in.
mern-skeleton/server/routes/user.routes.js:
router.param('userId', userCtrl.userByID)
The userByID controller function uses the value in the :userId param to query the database by _id, and load the matching user's details.
mern-skeleton/server/controllers/user.controller.js:
const userByID = (req, res, next, id) => { User.findById(id).exec((err, user) => { if (err || !user) return res.status('400').json({ error: "User not found" }) req.profile = user next() })}
If a matching user is found ...
