Why is res.header not being set?

Question

Node.js

Can't display res.header('Content-type', 'application/json')
Tried

app.use((req, res, next) => {
  res.header('Content-type', 'application/json')
  next()
})

and

app.get('/pets', (req, res) => {
    res.header('Content-type', 'application/json')
}

Answer 1

res.set("Content-Type", "application/json");

// response.json(); // Можно и так отправить json без явного указания типа