When you listen for connections on a route in Express, the callback function will be invoked on every network call with a Request object instance and a Response object instance.
Example:
app.get('/', (req, res) => res.send('Hello World!'))
Here we used the Response.send() method, which accepts any string.
You can send JSON to the client by using Response.json(), a useful method.
It accepts an object or array, and converts it to JSON before sending it:
res.json({ username: 'Flavio' })