Code Quality Skill

Provides general code quality and best practices guidance applicable across languages and frameworks. Focuses on Linting, Testing, and Type Safety.

For architecture-specific patterns (N+1 queries, Soft Deletes, etc.), refer to:

Pre-Submission Checklist

Before marking any task as complete:

pnpm --filter <app> lint passes (no ESLint rule disables).
pnpm --filter <app> typecheck passes.
pnpm --filter <app> build succeeds — mandatory, stricter than typecheck alone.
pnpm --filter <app> test passes (new features have tests).
Avoid any / unknown types (maintain strict type safety).
Remove console.log statements (use a dedicated logger).
Ensure error messages are clear and actionable.

Linting

We use ESLint with strict rules.

Command: pnpm lint (or pnpm lint -- --fix)
Rule: NEVER disable rules with eslint-disable. Fix the underlying issue.

Common Fixes:

@typescript-eslint/no-explicit-any: Define a proper interface/DTO.
no-unused-vars: Remove the variable or prefix with _.
no-console: Inject a Logger service.

Testing

All new features require tests.

Unit Tests: Test individual classes (Services, Utils) with mocked dependencies.
Integration Tests: Test interactions (Repositories) with real database/services.

Example (Unit Test):

describe('UserService', () => {
  it('should return user when found', async () => {
    // 1. Arrange (Mock dependencies)
    const mockRepo = { findByUid: vi.fn().mockResolvedValue(user) };  // Vitest — not jest.fn()
    const service = new UserService(mockRepo as any);
 
    // 2. Act
    const result = await service.getUser('u_1');
 
    // 3. Assert
    expect(result).toEqual(user);
    expect(mockRepo.findByUid).toHaveBeenCalledWith('u_1');
  });
});

TypeScript Type Safety

Strict mode is enforced.

❌ Avoid any / unknown:

// BAD
const data: any = req.body;

// GOOD
const data: CreateUserDto = req.body;

✅ Use DTOs and Interfaces: Always define shapes for inputs and outputs.
✅ Trust the Compiler: If it compiles, it should likely run (if types are accurate).

Common Anti-Patterns (General)

Ignoring Lint Errors: Address them immediately.
Logic in Controllers: Controllers should only handle HTTP req/res. Move logic to Services.
Hardcoded Strings/Magic Numbers: Use constants or enums.
Complex Conditionals: Break down complex if/else blocks into helper methods.
Catch-All Error Handling: Avoid just using console.error. Handle specific errors or let global filters handle them.

Related Skills

Database Patterns: N+1 queries, Soft Deletes, Bulk Operations.
Service Pattern NestJS: Business logic errors, Transactions.
Repository Pattern NestJS: Data access rules.
Backend Controller Pattern NestJS: NestJS-specific controller rules.
Frontend Code Quality: React/Frontend specific patterns.