How to test asynchronous code in Jest? What methods are available for handling Promises and callbacks?

Jest provides multiple methods for testing asynchronous code:

1. Using Callbacks:

javascript
test('async callback', (done) => {
  fetchData(callback => {
    expect(callback).toBe('data');
    done();
  });
});

2. Using Promises:

javascript
test('async promise', () => {
  return fetchData().then(data => {
    expect(data).toBe('data');
  });
});

// Or using async/await
test('async await', async () => {
  const data = await fetchData();
  expect(data).toBe('data');
});

3. Using resolves and rejects:

javascript
test('promise resolves', () => {
  return expect(fetchData()).resolves.toBe('data');
});

test('promise rejects', () => {
  return expect(fetchData()).rejects.toThrow('error');
});

4. Testing Timers:

javascript
jest.useFakeTimers();

test('timer callback', () => {
  const callback = jest.fn();
  setTimeout(callback, 1000);
  jest.runAllTimers();
  expect(callback).toHaveBeenCalled();
});

Best Practices:

• Prefer async/await for clearer code
• Ensure returning Promise or calling done
• Use resolves and rejects to simplify assertions
• Use jest.useFakeTimers() when testing timers
• Set reasonable timeout (default is 5 seconds)