Routes become much more powerful when they can be used dynamically. Express servers provide this functionality with named route parameters. Parameters are route path segments that begin with : in their Express route definitions. They act as wildcards, matching any text at that path segment. For example /monsters/:id will match both/monsters/1 and /monsters/45.

Express parses any parameters, extracts their actual values, and attaches them as an object to the request object: req.params. This object’s keys are any parameter names in the route, and each key’s value is the actual value of that field per request.

const monsters = { 
  hydra: { height: 3, age: 4 }, 
  dragon: { height: 200, age: 350 } 
};
// GET /monsters/hydra
app.get('/monsters/:name', (req, res, next) => {
  console.log(req.params); // { name: 'hydra' }
  res.send(monsters[req.params.name]);
});

In this code snippet, a .get() route is defined to match /monsters/:name path. When a GET request arrives for /monsters/hydra, the callback is called. Inside the callback, req.params is an object with the key name and the value hydra, which was present in the actual request path.

The appropriate monster is retrieved by name (the object key) from the monsters object and sent back to the client with res.send().