write-tests

Original：🇺🇸 English

Translated

Add test coverage to existing code with correct mock patterns. Use when adding tests to untested modules, writing regression tests for bugs, or user asks to test a specific file. Handles mockReset:true, vi.hoisted(), forwarding pattern, and test app builder utilities.

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Sourceacedergren/agentic-tools

Added on2026-02-20

NPX Install

npx skill4agent add acedergren/agentic-tools write-tests

SKILL.md Content

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Write Tests for Existing Code

Before Writing, Ask Yourself

Module type? Route handler, repository, plugin, utility, or service — each has a different mock strategy
Blast radius? Does this module have side effects (DB writes, API calls) that need isolation?
Nearest test file? Find the closest
```
*.test.ts
```
and match its structure exactly

Mock Strategy by Module Type

Module Type	Strategy
Route handler	Test app builder + session simulation + `app.inject()`
Repository	Mock DB connection + counter-based `execute`
Framework plugin	Real framework instance + selective dependency mocks
Pure utility	No mocks — test inputs/outputs directly
Service w/ DI	Mock injected deps via forwarding pattern

Mock Setup (mockReset: true)

If your test runner uses

mockReset: true

, most examples from the internet will silently fail.

typescript

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.

NEVER

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

Metacognitive Rule

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

Run

bash

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