ts-testinglisted

# TypeScript Testing Tests that use `as any` to satisfy the type checker aren't testing the types — they're hiding them. The goal is tests that are type-safe at the boundary, test the public interface rather than internals, and give meaningful errors when types change. ## Jest setup for TypeScript Use `ts-jest` or Vitest's native TypeScript support. Avoid transpile-only setups (like `babel-jest` without type checking) for type-related tests — they skip type errors. **Vitest** (preferred for new projects): ```ts // vitest.config.ts import { defineConfig } from 'vitest/config'; export default defineConfig({ test: { globals: true } }); ``` **Jest with ts-jest**: ```json // jest.config.json { "preset": "ts-jest", "testEnvironment": "node" } ``` ## Type-safe mocks with `jest.fn` Always provide the type parameter to `jest.fn` — don't let it default to `jest.Mock` (which is effectively `any`). ```ts // Bad: loses type information const fetchUser = jest.fn(); // Good: typed mock const fetchUser = jest.fn<Promise<User>, [string]>(); fetchUser.mockResolvedValue({ id: '1', name: 'Alice' }); ``` For mocking module-level functions, use `jest.mocked()` (Jest 27+) or `vi.mocked()` (Vitest) instead of casting: ```ts import { sendEmail } from './email'; jest.mock('./email'); const mockSendEmail = jest.mocked(sendEmail); mockSendEmail.mockResolvedValue(undefined); ``` `jest.mocked()` returns the function with its mock type overlaid — no `as any` needed. ## Mocking interfaces and