registration(req, res) {
  if (!req.user) return res.status(401).send('Registration failed');

  const { user } = req;
  return res.status(201).json({ user });
},

const { expect } = require('chai');
const sinon = require('sinon');
const authController = require(...);

describe('authController', () => {
  const USER = {
  email: 'test@test.com',
  password: 'Test12345',
  confirm: 'Test12345',
  username: 'Test',
};

it('it should send user data with email: test@test.com', () => {
  const req = { user: USER };
  const res = {
    status: sinon.stub().returnsThis(),
    json: sinon.spy(),
  };

  console.log('RES: ', res); // об этом чуть ниже
  authController.registration(req, res);
  expect(res.json).to.equal(USER);
});

expect(res.json.calledWith(USER)).to.equal(true);