Best Practices Guide

Universal code quality guidelines applicable to any language or framework.

Context: If .ai-factory/ARCHITECTURE.md exists, follow its folder structure, dependency rules, and module boundaries alongside these guidelines.

Read .ai-factory/skill-context/aif-best-practices/SKILL.md — MANDATORY if the file exists.

This file contains project-specific rules accumulated by $aif-evolve from patches, codebase conventions, and tech-stack analysis. These rules are tailored to the current project.

How to apply skill-context rules:

• Treat them as project-level overrides for this skill's general instructions
• When a skill-context rule conflicts with a general rule written in this SKILL.md, the skill-context rule wins (more specific context takes priority — same principle as nested CLAUDE.md files)
• When there is no conflict, apply both: general rules from SKILL.md + project rules from skill-context
• Do NOT ignore skill-context rules even if they seem to contradict this skill's defaults — they exist because the project's experience proved the default insufficient
• CRITICAL: skill-context rules apply to ALL outputs of this skill — including the recommendations, examples, and checklists you present. If a skill-context rule says "best practices MUST prioritize X" or "examples MUST follow convention Y" — you MUST comply. Presenting guidance that contradicts skill-context rules is a bug.

Enforcement: After generating any output artifact, verify it against all skill-context rules. If any rule is violated — fix the output before presenting it to the user.

Quick Reference

• $aif-best-practices — Full overview
• $aif-best-practices naming — Naming conventions
• $aif-best-practices structure — Code organization
• $aif-best-practices errors — Error handling
• $aif-best-practices testing — Testing practices
• $aif-best-practices review — Code review checklist

Naming Conventions

Variables & Functions

✅ Good                          ❌ Bad
─────────────────────────────────────────────
getUserById(id)                  getUser(i)
isValidEmail                     checkEmail
maxRetryCount                    max
calculateTotalPrice              calc
handleSubmit                     submit

Rules:

• Use descriptive names that reveal intent
• Avoid abbreviations (except universally known: id, url, api)
• Boolean variables: is, has, can, should prefix
• Functions: verb + noun (fetchUser, validateInput)
• Constants: SCREAMING_SNAKE_CASE
• Classes/Types: PascalCase
• Variables/functions: camelCase (JS/TS/PHP) or snake_case (Python/Rust)

Files & Directories

✅ Good                          ❌ Bad
─────────────────────────────────────────────
user-service.ts                  userService.ts (inconsistent)
UserRepository.ts                user_repository.ts (mixed)
/components/Button/              /Components/button/
/services/auth/                  /Services/Auth/

Rules:

• One convention per project (kebab-case or PascalCase for files)
• Directories: lowercase with hyphens
• Test files: *.test.ts or *.spec.ts (consistent)
• Index files: only for re-exports, not logic

Code Structure

Function Design

// ✅ Good: Single responsibility, clear inputs/outputs
function calculateDiscount(price: number, discountPercent: number): number {
  if (discountPercent < 0 || discountPercent > 100) {
    throw new Error('Discount must be between 0 and 100');
  }
  return price * (1 - discountPercent / 100);
}

// ❌ Bad: Multiple responsibilities, side effects
function processOrder(order) {
  validateOrder(order);           // validation
  order.discount = getDiscount(); // mutation
  saveToDatabase(order);          // persistence
  sendEmail(order.user);          // notification
  return order;
}

// ✅ Good: PHP with type declarations
function calculateDiscount(float $price, float $discountPercent): float
{
    if ($discountPercent < 0 || $discountPercent > 100) {
        throw new InvalidArgumentException('Discount must be between 0 and 100');
    }
    return $price * (1 - $discountPercent / 100);
}

Rules:

• Single Responsibility: one function = one job
• Max 20-30 lines per function
• Max 3-4 parameters (use object for more)
• No side effects in pure functions
• Early returns for guard clauses

Module Organization

feature/
├── index.ts          # Public exports only
├── types.ts          # Types and interfaces
├── constants.ts      # Constants
├── utils.ts          # Pure utility functions
├── hooks.ts          # React hooks (if applicable)
├── service.ts        # Business logic
└── repository.ts     # Data access

Rules:

• Group by feature, not by type
• Clear public API via index.ts
• Internal modules prefixed with _ or in internal/
• Avoid circular dependencies

Error Handling

Do's and Don'ts

// ✅ Good: Specific errors, meaningful messages
class UserNotFoundError extends Error {
  constructor(userId: string) {
    super(`User not found: ${userId}`);
    this.name = 'UserNotFoundError';
  }
}

async function getUser(id: string): Promise<User> {
  const user = await db.users.find(id);
  if (!user) {
    throw new UserNotFoundError(id);
  }
  return user;
}

// ❌ Bad: Generic errors, swallowed exceptions
async function getUser(id) {
  try {
    return await db.users.find(id);
  } catch (e) {
    console.log(e);  // Swallowed!
    return null;     // Hides the problem
  }
}

Rules:

• Create specific error classes for domain errors
• Never swallow exceptions without logging
• Log errors with context (user ID, request ID, etc.)
• Use error boundaries at system edges
• Return Result types for expected failures (optional)

Error Messages

✅ Good: "Failed to create user: email 'test@example.com' already exists"
❌ Bad: "Error occurred"
❌ Bad: "Something went wrong"

Testing Practices

Test Structure (AAA Pattern)

describe('calculateDiscount', () => {
  it('should apply percentage discount to price', () => {
    // Arrange
    const price = 100;
    const discount = 20;

    // Act
    const result = calculateDiscount(price, discount);

    // Assert
    expect(result).toBe(80);
  });

  it('should throw for invalid discount percentage', () => {
    expect(() => calculateDiscount(100, -10)).toThrow();
    expect(() => calculateDiscount(100, 150)).toThrow();
  });
});

Rules:

• One assertion concept per test
• Descriptive test names: "should [expected behavior] when [condition]"
• Test behavior, not implementation
• Use factories/fixtures for test data
• Avoid testing private methods directly

Test Coverage Priorities

1. Critical business logic      ████████████ Must have
2. Edge cases and boundaries    ████████░░░░ Important
3. Integration points           ██████░░░░░░ Important
4. Happy paths                  ████░░░░░░░░ Basic
5. UI components                ██░░░░░░░░░░ Optional

Code Review Checklist

Before Requesting Review

• Self-reviewed the diff
• Tests pass locally
• No debug code (console.log, debugger)
• No commented-out code
• Updated documentation if needed
• Commit messages are clear

Reviewer Checklist

• Correctness: Does it do what it claims?
• Edge cases: What could go wrong?
• Security: Any vulnerabilities? (see $aif-security-checklist)
• Performance: Any obvious bottlenecks?
• Readability: Can I understand it in 5 minutes?
• Tests: Are critical paths covered?
• Consistency: Follows project conventions?

Review Comments

✅ Good feedback:
"This could throw if `user` is null. Consider adding a null check
or using optional chaining: `user?.profile?.name`"

❌ Bad feedback:
"This is wrong"
"I don't like this"
"Why did you do it this way?"

Quick Rules Summary

Artifact Ownership and Config Policy

• Primary ownership: none. This skill is advisory and reference-only.
• Write policy: do not create or modify project artifacts by default.
• Config policy: config-agnostic by design. Follow repository context, .ai-factory/ARCHITECTURE.md, and skill-context overrides instead of reading config.yaml.