user.test.js

import { fetchUser } from '../../firebase/users/read';

// Mock user to return when fetched.
const mockUser = {
  name: 'Bob',
};

// Mocked Firebase fetch user.
export const fetchUser = (doResolve, returnValue) => () => new Promise(
  (resolve, reject) => process.nextTick(() => doResolve ? resolve(returnValue) : reject(returnValue))
);

jest.mock('../../firebase/users/read', () => ({
  fetchUser: fetchUser(true, mockUser),
}));

describe('user module', () => {
  describe('#fetchUser', () => {
    expect.assertions(2);
    return fetchUser(1).then(user => {
      expect(user).toBeDefined();
      expect(user.name).toBe(mockUser.name);
    });
  });
});