Skill: Write Tests

Objective

Add or expand tests for existing code. This skill creates or extends test files with meaningful cases, edge cases, and mocks.

Current Status

• Testing Framework: Vitest 4.x (configured and active)
• Component Testing: @testing-library/svelte 5.x
• DOM Environment: happy-dom
• Coverage: V8 provider, 80%+ threshold on src/lib/
• Test file naming: *.test.ts

Non-Goals

• Does NOT implement new production code
• Does NOT refactor production logic
• Does NOT change function signatures for the sake of testability without agreement
• Does NOT set up E2E tests (Playwright not configured)

Tier Classification

Tier: 2 - Standard

Reasoning: Writing tests requires moderate reasoning (understanding behavior, edge cases, mocks). Scope typically 1-10 files, 100-500 LOC; standard development work.

Inputs

Required Parameters

• $TARGET: File(s) or module to add tests for (e.g., src/lib/i18n.ts)
• $SCOPE: What to cover (e.g., "unit for props", "interaction tests", "edge cases for null")

Optional Parameters

• $EXISTING: Path to existing test file to extend

Prerequisites

• Target code is stable (no pending refactors)
• Test fixtures understood (tests/fixtures/)
• Vitest config reviewed if testing new module types

Test Infrastructure

Configuration

• Config: vitest.config.ts at project root
• Environment: happy-dom
• Path aliases: @/ maps to src/ (matches tsconfig)
• Svelte: @sveltejs/vite-plugin-svelte with hot: false
• Browser resolve: conditions: ['browser'] for Svelte 5 component tests
• astro:content mock: tests/mocks/astro-content.ts (aliased in vitest config)

Test File Structure

tests/
├── unit/
│   ├── lib/                            # Utility function tests
│   │   ├── i18n.test.ts
│   │   ├── markdown-for-agents.test.ts
│   │   └── translations.test.ts
│   └── components/                     # Svelte component tests
├── fixtures/                           # Shared mock data
├── helpers/
│   └── setup.ts                        # Test setup (jest-dom matchers)
└── mocks/
    └── astro-content.ts                # Mock for astro:content virtual module

Key Patterns

Utility function tests:

import { describe, expect, it } from 'vitest';
import { getUrlPrefix } from '@/lib/i18n';

describe('getUrlPrefix', () => {
  it('returns /es for Spanish', () => {
    expect(getUrlPrefix('es')).toBe('/es');
  });
});

Svelte component tests:

import { render, screen } from '@testing-library/svelte';
import { describe, expect, it } from 'vitest';
import MethodologyCard from '@/components/MethodologyCard.svelte';

const doc = { data: { title: 'Introduction', order: 1, lang: 'en' } };

describe('MethodologyCard', () => {
  it('renders doc title', () => {
    render(MethodologyCard, { props: { doc: doc as never } });
    expect(screen.getByText('Introduction')).toBeDefined();
  });
});

Important notes:

• Use as never cast for mock entries passed to functions expecting a CollectionEntry
• Svelte 5 components require resolve.conditions: ['browser'] in vitest config
• Do NOT test async functions that depend on astro:content directly — mock the collection instead

Steps

Step 1: Understand Target

• Read the target file(s) and identify public behavior to test
• Check for existing tests in tests/unit/
• Review testing patterns in existing test files

Step 2: Plan Cases

• List unit cases (happy path, edge cases, errors)
• Identify dependencies to mock
• Decide: new file or extend existing test

Step 3: Implement Tests

• Create or update test file in tests/unit/
• Use Vitest (describe, it, expect)
• Import fixtures from tests/fixtures/ when applicable
• Keep tests focused; avoid testing implementation details

Step 4: Validate

pnpm run test           # Run tests
pnpm run biome:check    # Lint check

Output Format

Success Output

## Tests Added/Updated

### Target
{What was tested}

### Files
- `tests/.../foo.test.ts`: {created|updated} — {brief description}

### Coverage
- {list of cases covered}

### Validation
- pnpm run test: pass
- pnpm run biome:check: pass

### Commit Message
test: add|expand tests for {target}

Guardrails

Scope Limits

• Maximum files: 10 (test files + any small test helpers)
• Maximum LOC: 500 (test code)
• Naming: *.test.ts

Stop Conditions

Stop and escalate if:

• Target is auth/security-critical (coordinate with team)
• Requires new test infrastructure not in project
• Scope exceeds 10 files or 500 LOC
• Need to mock astro:content beyond the existing stub

Definition of Done

• New or updated test files
• All tests pass (pnpm run test)
• pnpm run biome:check passes
• Tests are meaningful (not only coverage padding)

Escalation Conditions

Escalate to Tier 3 if: integration/E2E design needed, or test architecture decisions.

Related

• quick-fix - Small fixes
• reviewer - Review test quality
• docs/TESTING_GUIDE.md - Full testing conventions and setup

Changelog

• 2026-02-17: Updated from placeholder to actual Vitest setup. Removed "NOT CONFIGURED" status. Added real patterns, config details, and Svelte 5 compatibility notes.