MSW

MSW (Mock Service Worker) provides API mocking that works everywhere. By intercepting at the network level, the same mocks work in tests, Storybook, and development without code changes.

Key Features:

• Network Level - Intercepts actual requests, not function calls
• Reusable Mocks - Same handlers for tests, stories, and dev
• Type Safe - Full TypeScript support
• REST & GraphQL - Both API styles supported

Why MSW:

• Real Network - Your code makes real fetch calls
• No Code Changes - Mock without modifying application code
• Consistent - Same mocks across all environments
• DevTools Compatible - Requests appear in Network tab

MSW capabilities:

• Request Matching - Match by URL, method, headers
• Dynamic Responses - Generate responses based on requests
• Error Scenarios - Test error handling easily
• Delayed Responses - Test loading states

MSW has become the standard for API mocking in frontend development. The network-level approach means your tests and stories exercise real code paths, catching issues that function-level mocks would miss.