Learn the concept
Testing Asynchronous Code
Jest supports testing async code with: async/await (recommended), returning Promises, or using the done callback. For async/await, mark test as async and use await. Always ensure async tests complete properly or they'll timeout.
Methods for Async Testing:
async/await (Recommended)
Returning Promises
done Callback
Testing Rejections:
expect(promise).rejects.toThrow()try/catch with expect.assertions()// Function to test
async function fetchUser(id) {
const response = await fetch(`/api/users/${id}`);
if (!response.ok) {
throw new Error('User not found');
}
return response.json();
}
describe('fetchUser', () => {
// Async/await approach (recommended)
test('fetches user successfully', async () => {
const user = await fetchUser(1);
expect(user).toHaveProperty('name');
expect(user.id).toBe(1);
});
// Testing rejection
test('throws error for invalid user', async () => {
await expect(fetchUser(999)).rejects.toThrow('User not found');
});
// Alternative: try/catch
test('handles error with try/catch', async () => {
expect.assertions(1); // Ensure assertion runs
try {
await fetchUser(999);
} catch (error) {
expect(error.message).toBe('User not found');
}
});
});Using `async/await` with `jest-dom/extend-expect` to test the hook's behavior, ensuring it correctly handles loading states, successful data retrieval, and error conditions when interacting with asynchronous operations.
Employing promises or the `done` callback pattern to ensure that the test waits for the asynchronous message to be sent/received and then asserts on the expected state changes.
Structuring tests with `async/await` to simulate user interactions and await the completion of each asynchronous step, asserting on UI changes and data persistence at various points in the flow.
Create a simple JavaScript function that simulates fetching data from an API using `setTimeout` to mimic latency. Write Jest tests for this function using `async/await` to verify successful data retrieval and error handling.
Develop a utility function that returns a Promise. Write Jest tests that use `.resolves` and `.rejects` matchers to assert the successful resolution and error rejection of the promise.
Stripe's frontend SDKs and backend services deal extensively with asynchronous operations due to network requests for payment processing. Their test suites use `async/await` and promise-based assertions to rigorously test the reliability and correctness of these critical asynchronous flows.
Netflix's highly dynamic user interface involves numerous asynchronous interactions, from fetching movie recommendations to handling user preferences. Their testing infrastructure heavily relies on Jest's async testing capabilities to ensure these interactions are seamless and error-free.