Simulación de módulos nativos y código asíncrono

The Native Module Problem in Jest

Manual Mocks in __mocks__ Folder

// __mocks__/@react-native-async-storage/async-storage.js
const storage = {};

export default {
  getItem: jest.fn((key) => Promise.resolve(storage[key] ?? null)),
  setItem: jest.fn((key, value) => {
    storage[key] = value;
    return Promise.resolve();
  }),
  removeItem: jest.fn((key) => {
    delete storage[key];
    return Promise.resolve();
  }),
  clear: jest.fn(() => {
    Object.keys(storage).forEach((k) => delete storage[k]);
    return Promise.resolve();
  }),
  getAllKeys: jest.fn(() => Promise.resolve(Object.keys(storage))),
};

Using the Official Mock Package

// jest.config.js
module.exports = {
  preset: 'jest-expo',
  moduleNameMapper: {
    '@react-native-async-storage/async-storage':
      '@react-native-async-storage/async-storage/jest/async-storage-mock',
  },
  setupFilesAfterFramework: [
    '@testing-library/jest-native/extend-expect',
  ],
};

Mocking expo-location

jest.mock('expo-location', () => ({
  requestForegroundPermissionsAsync: jest.fn(),
  getCurrentPositionAsync: jest.fn(),
  PermissionStatus: { GRANTED: 'granted', DENIED: 'denied' },
}));

import * as Location from 'expo-location';

it('shows coordinates when permission is granted', async () => {
  Location.requestForegroundPermissionsAsync
    .mockResolvedValue({ status: 'granted' });
  Location.getCurrentPositionAsync
    .mockResolvedValue({ coords: { latitude: 37.78, longitude: -122.43 } });

  const { findByText } = render(<LocationScreen />);

  await findByText('37.78, -122.43');
});

Mocking the Fetch API

beforeEach(() => {
  global.fetch = jest.fn();
});

afterEach(() => {
  jest.restoreAllMocks();
});

it('displays fetched posts in a list', async () => {
  const mockPosts = [{ id: '1', title: 'Hello World' }];

  global.fetch.mockResolvedValue({
    ok: true,
    json: () => Promise.resolve(mockPosts),
  });

  const { findByText } = render(<PostFeed />);

  await findByText('Hello World');
  expect(global.fetch).toHaveBeenCalledWith(
    'https://api.example.com/posts'
  );
});

Mocking Axios

import axios from 'axios';
import MockAdapter from 'axios-mock-adapter';

const mock = new MockAdapter(axios);

afterEach(() => mock.reset());

it('fetches and displays user profile', async () => {
  mock.onGet('/api/users/123').reply(200, {
    id: '123',
    name: 'Alice',
    email: 'alice@test.com',
  });

  const { findByText } = render(<ProfileScreen userId='123' />);

  await findByText('Alice');
  await findByText('alice@test.com');
});

Testing Async Hooks with renderHook

import { renderHook, act, waitFor } from '@testing-library/react-native';
import { useUserProfile } from '../src/hooks/useUserProfile';

it('fetches the user profile on mount', async () => {
  global.fetch = jest.fn().mockResolvedValue({
    ok: true,
    json: () => Promise.resolve({ id: '1', name: 'Bob' }),
  });

  const { result } = renderHook(() => useUserProfile('1'));

  // Initially loading
  expect(result.current.loading).toBe(true);

  // Wait for the fetch to complete
  await waitFor(() => expect(result.current.loading).toBe(false));

  expect(result.current.user?.name).toBe('Bob');
});

Mocking AsyncStorage in Tests

import AsyncStorage from '@react-native-async-storage/async-storage';

it('loads dark mode preference on mount', async () => {
  // Arrange: storage returns 'dark' for theme key
  AsyncStorage.getItem.mockResolvedValue('dark');

  const { findByText } = render(<SettingsScreen />);

  // Assert theme applied
  await findByText('Dark Mode: ON');
  expect(AsyncStorage.getItem).toHaveBeenCalledWith('theme');
});

it('saves the new theme when toggle is pressed', async () => {
  AsyncStorage.getItem.mockResolvedValue('light');
  const { getByTestId, waitFor } = render(<SettingsScreen />);

  fireEvent(getByTestId('theme-toggle'), 'valueChange', true);

  await waitFor(() =>
    expect(AsyncStorage.setItem).toHaveBeenCalledWith('theme', 'dark')
  );
});

Testing with waitFor and findBy

import { render, fireEvent, waitFor } from '@testing-library/react-native';

it('shows error message when API returns 400', async () => {
  global.fetch = jest.fn().mockResolvedValue({
    ok: false,
    status: 400,
    json: () => Promise.resolve({ message: 'Invalid email' }),
  });

  const { getByRole, findByText } = render(<SignUpForm />);
  fireEvent.press(getByRole('button', { name: 'Create Account' }));

  // Wait for the error to appear after the API response
  expect(await findByText('Invalid email')).toBeTruthy();

  // Alternatively:
  await waitFor(() => {
    expect(getByText('Invalid email')).toBeTruthy();
    expect(getByRole('button', { name: 'Create Account' })).not.toBeDisabled();
  });
});

Mocking timers for Debounced Inputs

it('triggers search after 500ms debounce', async () => {
  jest.useFakeTimers();
  const mockSearch = jest.fn().mockResolvedValue([]);
  const { getByPlaceholderText } = render(<DebouncedSearch onSearch={mockSearch} />);

  fireEvent.changeText(getByPlaceholderText('Search...'), 'react');

  // Not yet called — debounce delay not elapsed
  expect(mockSearch).not.toHaveBeenCalled();

  // Advance time past the debounce window
  await act(async () => {
    jest.advanceTimersByTime(500);
  });

  expect(mockSearch).toHaveBeenCalledWith('react');
  jest.useRealTimers();
});

Test Setup Files for Global Mocks

// jest-setup.ts (referenced in jest.config.js setupFilesAfterFramework)
import '@testing-library/jest-native/extend-expect';

// Mock AsyncStorage globally
jest.mock('@react-native-async-storage/async-storage',
  () => require('@react-native-async-storage/async-storage/jest/async-storage-mock')
);

// Mock analytics to avoid network calls in tests
jest.mock('@segment/analytics-react-native', () => ({
  createClient: jest.fn(() => ({ track: jest.fn(), identify: jest.fn() })),
}));

// Silence console.error for expected React warnings in tests
const consoleError = console.error;
console.error = (...args) => {
  if (String(args[0]).includes('Warning:')) return;
  consoleError(...args);
};

Quick Check

Lesson Recap