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:

• Database Patterns

• Service Patterns

• Repository Patterns

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.