How do you return JSON from an Express endpoint?

Set the application/json content type and send a stringified object — or just call res.json(), which does both.

router.get('/', function(req, res) {
    res.type('application/json');

    const response = {};

    if ('firstName' in req.query && req.query.firstName !== '') {
        response.message = 'Hello ' + req.query.firstName;
    } else {
        response.error = "Parameter 'firstName' not set or empty";
        res.status(400);
    }

    res.send(JSON.stringify(response));
});

Request: GET /hello?firstName=Anna Response: {"message":"Hello Anna"}

Shortcut: Express has res.json() which combines type setting and stringify:

res.json({ message: 'Hello Anna' });