write-tests

Context

• Target file: $1
• Runner override (optional): $2
• Project manifest (detect runner, scripts, and deps from here):

!cat package.json

Task

Write a thorough, idiomatic test suite for $1. If no file path was given in the arguments above, ask for one and stop.

Follow this procedure:

1. Detect the toolchain — don't assume

From the manifest above (and config files if needed), determine the runner and libraries already in use. Respect $2 if the user named a runner; otherwise infer:

• Runner: vitest, jest, node:test, mocha, ava, or @playwright/test. Check devDependencies and the test script.
• Helpers in play: @testing-library/react + @testing-library/jest-dom (React), supertest (Express/HTTP), @nestjs/testing (NestJS), msw (network mocking).
• Config: vitest.config.*, jest.config.*, tsconfig paths/aliases, setupTests files.

If the project has no test runner installed, do not invent one silently: state what you'd add (default to Vitest for Vite/Next/library code, Jest where it's already the ecosystem norm) and ask before adding a dependency.

2. Mirror existing conventions

Find sibling tests with Glob/Grep (**/*.{test,spec}.{ts,tsx,js,jsx}) and read one or two. Match their file location (__tests__/ vs co-located *.test.ts), import style, naming, and setup/teardown patterns. New tests should look like they were written by whoever wrote the existing ones.

3. Understand the unit under test

Read $1 in full. Identify every exported function/component/class, its inputs, outputs, side effects, dependencies to mock (network, DB, clock, randomness, filesystem), and the branches that need coverage.

4. Write meaningful tests

• Cover behavior, not implementation. Assert observable outputs and effects; don't pin internal calls unless the contract is the call itself.
• Per unit, cover: the happy path, boundary/edge cases (empty, zero, negative, large, unicode), and error/failure paths (throws, rejections, invalid input).
• Structure each test Arrange–Act–Assert with a description that reads as a behavior (it("returns 401 when the token is missing")).
• Determinism is non-negotiable: fake timers for time, seed or stub randomness, no real network/DB/filesystem — mock at the boundary (msw, supertest, in-memory fakes). No sleep, no ordering assumptions, no shared mutable state between tests.
• Stack specifics:
  • React (Testing Library): query by role/label/text, never by test-id unless unavoidable; assert what the user sees; use userEvent for interaction; wrap async UI in findBy/waitFor. Don't test internal state.
  • Express: drive HTTP with supertest against the app instance; assert status, body, and headers; cover auth and validation failures.
  • NestJS: build a Test.createTestingModule, provide mocked providers, and test services in isolation; for controllers, mock the service layer.
• Keep each test independent and runnable in any order.