/write-tests

Write Tests for Existing Code

When to Use

Load this skill when the user request matches the frontmatter description for Write Tests for Existing Code.

NEVER

• NEVER chain mockResolvedValueOnce when mockReset: true — the chain clears between tests. Use counter-based mockImplementation instead.
• NEVER define mock variables at module scope then reference them inside vi.mock() factories — hoisting creates a temporal dead zone. Use vi.hoisted() or globalThis registry.
• NEVER vi.importActual() for modules with side effects — use selective re-exports instead.
• NEVER test implementation details (private state, internal call order) — test observable behavior through the public API.
• NEVER copy mock patterns from other projects — check YOUR test runner config first (mockReset, mockClear, restoreMocks).
• NEVER modify source code — this skill writes tests only; production behavior is fixed.

Before Writing, Ask Yourself

• Module type? Each has a different mock strategy (see table below).
• Blast radius? Does this module have side effects (DB writes, API calls, filesystem) that need isolation?
• Nearest test file? Find the closest *.test.ts and match its exact mock structure — don't invent a new pattern.

Mock Strategy by Module Type

Mock Setup (mockReset: true)

If your test runner uses mockReset: true, most examples from the internet will silently fail — return values clear between tests.

const { mockFn } = vi.hoisted(() => ({
  mockFn: vi.fn(),
}));

vi.mock("./dependency", () => ({
  dependency: (...args: unknown[]) => mockFn(...args),
}));

beforeEach(() => {
  // MUST reconfigure here — mockReset clears return values between tests
  mockFn.mockResolvedValue(defaultResult);
});

For complex TDZ cases (multiple interdependent mocks), use the globalThis registry pattern:

vi.mock("./dep", () => {
  if (!(globalThis as any).__mocks) (globalThis as any).__mocks = {};
  const m = { dep: vi.fn() };
  (globalThis as any).__mocks.dep = m;
  return { dep: (...a: unknown[]) => m.dep(...a) };
});
// In tests: const mocks = (globalThis as any).__mocks;

Metacognitive Rule

If >3 tests fail on first run: STOP. Root cause is almost certainly a mock wiring issue affecting all tests — not individual test logic. Re-examine the mock setup holistically before fixing tests one by one.

Run

npx vitest run <test-file> --reporter=verbose

Arguments

• $ARGUMENTS: Path to the source file or module to cover
  • Example: /write-tests src/routes/admin/settings.ts
  • If empty: ask the user which file needs test coverage