Jest
Jest
Jest is the default JavaScript testing framework included with React Native, used for writing and running unit and integration tests for your application logic and components.
Jest is Pre-configured
New React Native projects come with Jest already configured, including a jest.config.js file and a preset specifically designed for React Native’s environment.
Writing a Basic Test
javascript
// sum.test.js
function sum(a, b) {
return a + b;
}
test('adds 1 + 2 to equal 3', () => {
expect(sum(1, 2)).toBe(3);
});
Running Tests
bash
npx jest
Using describe Blocks
javascript
describe('formatCurrency', () => {
test('formats whole numbers correctly', () => {
expect(formatCurrency(100)).toBe('$100.00');
});
test('formats decimals correctly', () => {
expect(formatCurrency(99.5)).toBe('$99.50');
});
});
Mocking Functions and Modules
javascript
jest.mock('../services/api');
import { fetchUsers } from '../services/api';
fetchUsers.mockResolvedValue([{ id: 1, name: 'John' }]);
Testing Asynchronous Code
javascript
test('fetches user data', async () => {
const data = await fetchUserData(1);
expect(data.name).toBe('John');
});
Generating Coverage Reports
bash
npx jest --coverage
Conclusion
Jest provides a robust, pre-configured foundation for writing unit and integration tests in React Native applications, forming the basis for the more specialized testing tools covered later in this section.
