Jest Mocking Not Working with ESM in Windsurf Project
Jest mocking doesn't work in your Windsurf-generated project that uses ES modules (ESM). jest.mock() calls are silently ignored, mocked modules still return real implementations, or you get errors about import statements not being mockable. Tests that should use mocked dependencies are hitting real APIs or databases.
This is one of the most common testing frustrations in modern JavaScript projects. Jest was designed for CommonJS (require), and its mocking system doesn't work the same way with ESM imports. Cascade generates test files with jest.mock() patterns that only work in CommonJS, leading to tests that appear to set up mocks correctly but actually use the real modules.
You might notice this when tests make real HTTP requests, modify your actual database, or fail with authentication errors — all signs that mocking isn't working and real dependencies are being used.
Error Messages You Might See
Common Causes
- jest.mock() hoisting doesn't work with ESM — In CommonJS, Jest hoists jest.mock() calls to the top of the file. With ESM, import statements are evaluated before any code runs, so mocks aren't in place when imports execute
- Missing babel/ts-jest transformation — Without proper transformation, import statements aren't converted to requires, and Jest's mock system can't intercept them
- Using import instead of require in mock setup — Mocked modules imported with static import get the real module, not the mock
- jest.config using wrong transform — The Jest configuration doesn't have the correct transform for .ts or .tsx files with ESM syntax
- Mock factory function returning wrong shape — The mock factory doesn't match the module's export shape (named exports vs default export)
How to Fix It
- Use jest.unstable_mockModule for ESM — Replace jest.mock() with jest.unstable_mockModule() and use dynamic import() after setting up mocks
- Configure ts-jest or babel-jest properly — Ensure your Jest config transforms ESM to CJS: transform: { '^.+\\.tsx?$': 'ts-jest' } with useESM: true in ts-jest config
- Use dependency injection instead — Refactor code to accept dependencies as parameters rather than importing them directly. This makes testing trivial regardless of module system
- Match mock shape to real module — For named exports, return an object with all exported names. For default exports, use { __esModule: true, default: mockFn }
- Consider Vitest as an alternative — Vitest has native ESM support and a Jest-compatible API. Migration is often straightforward and fixes ESM mocking issues
- Use manual mocks in __mocks__ directory — Create a __mocks__/module-name.ts file that Jest automatically uses. This approach works with both CJS and ESM
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Get HelpFrequently Asked Questions
Why doesn't jest.mock() work with ES modules?
jest.mock() relies on hoisting the mock setup before require() calls. With ESM, import statements are resolved at parse time before any code runs, so jest.mock() hasn't executed yet when imports are resolved. Use jest.unstable_mockModule() with dynamic import() instead.
Should I switch from Jest to Vitest?
If you're fighting ESM mocking issues, Vitest is worth considering. It has native ESM support, a nearly identical API to Jest, and handles module mocking natively. Migration is usually just changing imports and config, not rewriting tests.