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.

5. Verify

Run the suite with the detected runner (e.g. npx vitest run <file>, npx jest <file>) targeting only the new file. Fix failures by correcting the test or the mock: never by weakening an assertion to force a pass. If a test reveals a real bug in $1, stop and report it rather than writing the test to expect the buggy behavior.

6. Report

State the test file path, the runner used, what scenarios you covered (and any deliberately left out), the pass result, and anything the author should follow up on.