How to test error handling and exceptions in Jest? How to use toThrow and rejects?

Testing error handling and exceptions in Jest is crucial for ensuring code robustness:

1. Testing Synchronous Function Errors:

javascript
test('throws error when invalid input', () => {
  expect(() => divide(10, 0)).toThrow();
  expect(() => divide(10, 0)).toThrow('Division by zero');
  expect(() => divide(10, 0)).toThrow(/zero/);
});

2. Testing Promise Rejections:

javascript
test('rejects when API fails', async () => {
  await expect(fetchData()).rejects.toThrow('API Error');
});

// Or use .catch
test('rejects when API fails', async () => {
  await expect(fetchData()).rejects.toEqual({
    message: 'API Error'
  });
});

3. Testing Errors in Async Callbacks:

javascript
test('callback with error', (done) => {
  fetchData((err, data) => {
    expect(err).toBeInstanceOf(Error);
    expect(err.message).toBe('Network error');
    done();
  });
});

4. Testing Custom Error Types:

javascript
test('throws custom error', () => {
  expect(() => validate(null)).toThrow(ValidationError);
});

5. Using try-catch for Testing:

javascript
test('handles error gracefully', async () => {
  try {
    await riskyOperation();
    fail('Should have thrown an error');
  } catch (error) {
    expect(error).toBeInstanceOf(Error);
    expect(error.message).toContain('failed');
  }
});

6. Testing Error Boundaries (React):

javascript
test('ErrorBoundary catches errors', () => {
  const consoleSpy = jest.spyOn(console, 'error').mockImplementation(() => {});
  
  render(
    <ErrorBoundary>
      <ThrowingComponent />
    </ErrorBoundary>
  );
  
  expect(screen.getByText('Something went wrong')).toBeInTheDocument();
  consoleSpy.mockRestore();
});

Best Practices:

• Clearly test error messages and types
• Use toThrow() to match error messages or regex
• Test error handling logic, not just errors themselves
• Ensure cleanup of mocks and spies
• Test edge cases and invalid inputs