Why is res.header() not working in express? - node.js

Question

I am trying to authenticate a user for login. However, I don't manage to send a response header.
Login function
// POST /users/login
router.post('/users/login', async (req, res) => {
try {
const body = _.pick(req.body, ['email', 'password']);
const user = await User.findByCredentials(body.email, body.password);
const token = await user.generateAuthToken();
req.session['token'] = 'Bearer ' + token;
req.session['user'] = user;
// the following line does not seem to work properly
res.header('Authorization', 'Bearer ' + token).send(user);
} catch (err) {
res.status(401).json({ message: "Nutzerdaten sind nicht korrekt" });
}
});
Axios plugin
$axios.onResponse(res => {
console.log(res.headers) // does not return an authorization header
if (res.headers.authorization) {
store.commit('auth/SET_TOKEN', res.headers.authorization)
}
return res
})
console.log(res.headers)
'x-powered-by': 'Express',
vary: 'Origin',
'access-control-allow-credentials': 'true',
'content-type': 'application/json; charset=utf-8',
'content-length': '2',
etag: 'W/"2-l9Fw4VUO7kr8CvBlt4zaMCqXZ0w"',
date: 'Tue, 30 Jul 2019 12:35:00 GMT',
connection: 'keep-alive'
I would be very thankful for any kind of help!