You may have noticed in the previous exercise that our PUT route had no information about how to update the specified expression, just the id of which expression to update. It turns out that there was more information in the request in the form of a query string. Query strings appear at the end of the path in URLs, and they are indicated with a ? character. For instance, in /monsters/1?name=chimera&age=1, the query string is name=chimera&age=1 and the path is /monsters/1/

Query strings do not count as part of the route path. Instead, the Express server parses them into a JavaScript object and attaches it to the request body as the value of req.query. The key: value relationship is indicated by the = character in a query string, and key-value pairs are separated by &. In the above example route, the req.query object would be { name: 'chimera', age: '1' }.

const monsters = { '1': { name: 'cerberus', age: '4'  } };
// PUT /monsters/1?name=chimera&age=1
app.put('/monsters/:id', (req, res, next) => {
  const monsterUpdates = req.query;
  monsters[req.params.id] = monsterUpdates;
  res.send(monsters[req.params.id]);
});

Here, we have a route for updating monsters by ID. When a PUT /monsters/1?name=chimera&age=1 request arrives, our callback function is called and, we create a monsterUpdates variable to store req.query. Since req.params.id is '1', we replace monsters['1']‘s value with monsterUpdates . Finally, Express sends back the new monsters['1'].

When updating, many servers will send back the updated resource after the updates are applied so that the client has the exact same version of the resource as the server and database.