Contract Testing

Contract Testing Overview

Contract 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 contract 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.

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

Module Federation Cheatsheet

Key Module Federation settings related to contract testing.

How Contract Testing Works in Module Federation

Contract 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 Contract Testing

In production, contract 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

• Copying contract testing snippets without understanding what each line does.
• Skipping error handling and edge cases when wiring up contract testing.
• Leaving contract testing untested, so regressions slip into production.
• Over-engineering contract testing before you actually need the extra flexibility.

Key Takeaways

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