Unit Testing

Unit Testing Overview

Unit Testing lets you structure Module Federation work so it stays readable, testable, and easy to scale. Instead of ad-hoc code, you follow a clear pattern that other developers can recognise immediately.

The key is to keep unit testing focused and predictable. Start from the minimal example here, then layer in only the complexity your feature actually needs.

import { render, screen } from '@testing-library/react';
import ProductList from './ProductList';

test('remote component renders products', () => {
  render(<ProductList products={[{ id: 1, name: 'Book' }]} />);
  expect(screen.getByText('Book')).toBeInTheDocument();
});

Test exposed components in isolation the same way you test any local component.

Unit Testing Example

new ModuleFederationPlugin({
  name: 'app',
  filename: 'remoteEntry.js',
  exposes: { './Widget': './src/Widget' },
  remotes: { other: 'other@http://host/remoteEntry.js' },
  shared: { react: { singleton: true } },
});

• Start from a minimal Unit Testing example and grow it only as needed.
• Keep configuration explicit so Unit Testing behaves the same in every environment.
• Name things clearly so teammates understand your Unit Testing at a glance.
• Add tests around Unit Testing early to lock in expected behaviour.

Module Federation Cheatsheet

Key Module Federation settings related to unit testing.

How Unit Testing Works in Module Federation

Unit Testing builds on Module Federation's ability to load code from another independently built and deployed application at runtime. Each app can be a host, a remote, or both.

Test exposed components in isolation the same way you test any local component.

• Remotes expose modules through a remoteEntry.js manifest.
• Hosts declare remotes and import exposed modules dynamically.
• Shared dependencies are deduplicated, ideally as singletons.
• Each micro frontend builds and deploys on its own schedule.

Practical Guidance for Unit Testing

In production, unit testing needs careful version management and graceful failure handling. Align shared dependency versions and always render a fallback when a remote cannot load.

Common Mistakes

• Skipping error handling and edge cases when wiring up unit testing.
• Leaving unit testing untested, so regressions slip into production.
• Over-engineering unit testing before you actually need the extra flexibility.
• Ignoring documentation, which makes unit testing hard for the next developer to change.

Key Takeaways

• Unit Testing is a core part of working effectively with Module Federation.
• Start small and keep unit testing focused on a single responsibility.
• Apply consistent patterns so unit testing scales across your project.
• Test and document unit testing to keep it maintainable over time.