Code Quality

Overview

Principles and practices for writing maintainable, readable, and reliable code.

Clean Code Principles

Meaningful Names

// ❌ Cryptic names
const d = new Date();
const u = getU();
const arr = data.filter(x => x.s === 'a');

// ✅ Descriptive names
const currentDate = new Date();
const currentUser = getCurrentUser();
const activeUsers = users.filter(user => user.status === 'active');

// ❌ Hungarian notation (outdated)
const strName = 'John';
const arrItems = [];
const bIsActive = true;

// ✅ Let the type system handle types
const name = 'John';
const items: Item[] = [];
const isActive = true;

Functions

// ❌ Does too much
function processUserData(userId: string) {
  const user = db.findUser(userId);
  const orders = db.findOrders(userId);
  const total = orders.reduce((sum, o) => sum + o.amount, 0);
  sendEmail(user.email, `Your total: ${total}`);
  updateAnalytics(userId, total);
  return { user, orders, total };
}

// ✅ Single responsibility
function getUser(userId: string): User {
  return db.findUser(userId);
}

function getUserOrders(userId: string): Order[] {
  return db.findOrders(userId);
}

function calculateTotal(orders: Order[]): number {
  return orders.reduce((sum, o) => sum + o.amount, 0);
}

function sendOrderSummary(user: User, total: number): void {
  sendEmail(user.email, `Your total: ${total}`);
}

// ❌ Too many parameters
function createUser(name, email, age, role, department, manager, startDate) {}

// ✅ Use object parameter
interface CreateUserParams {
  name: string;
  email: string;
  age?: number;
  role: Role;
  department: string;
  managerId?: string;
  startDate: Date;
}

function createUser(params: CreateUserParams): User {}

Comments

// ❌ Redundant comment
// Increment counter by 1
counter++;

// ❌ Outdated comment (code changed, comment didn't)
// Returns the user's full name
function getUserEmail(user: User) {
  return user.email;
}

// ✅ Explains WHY, not WHAT
// Use binary search because the list is sorted and can have 100k+ items
const index = binarySearch(sortedItems, target);

// ✅ Warns about non-obvious behavior
// IMPORTANT: This function mutates the input array for performance reasons
function quickSort(arr: number[]): number[] {
  // ...
}

// ✅ TODO with context
// TODO(john): Remove after migration completes - tracking in JIRA-1234
const legacyAdapter = new LegacyAdapter();

SOLID Principles

Single Responsibility Principle

// ❌ Multiple responsibilities
class UserManager {
  createUser(data: UserData) { /* DB logic */ }
  validateEmail(email: string) { /* Validation logic */ }
  sendWelcomeEmail(user: User) { /* Email logic */ }
  generateReport(users: User[]) { /* Report logic */ }
}

// ✅ Single responsibility each
class UserRepository {
  create(data: UserData): User { /* DB logic */ }
  findById(id: string): User | null { /* DB logic */ }
}

class UserValidator {
  validateEmail(email: string): boolean { /* Validation */ }
  validatePassword(password: string): ValidationResult { /* Validation */ }
}

class EmailService {
  sendWelcomeEmail(user: User): void { /* Email logic */ }
}

class UserReportGenerator {
  generate(users: User[]): Report { /* Report logic */ }
}

Open/Closed Principle

// ❌ Must modify to add new payment methods
class PaymentProcessor {
  process(payment: P