// Partial<T> - All properties optional
type UpdateUserDto = Partial<User>;

// Pick<T, K> - Select specific properties
type UserPublic = Pick<User, 'id' | 'name' | 'email'>;

// Omit<T, K> - Exclude specific properties
type UserWithoutPassword = Omit<User, 'password' | 'salt'>;

// Record<K, V> - Map keys to values
type RolePermissions = Record<UserRole, Permission[]>;

// Extract<T, U> - Extract union members matching type
type StringKeys = Extract<keyof User, string>;

// Exclude<T, U> - Exclude union members matching type
type NonMethodKeys = Exclude<keyof User, Function>;

// Conditional return types based on input
type ApiResponse<T> = T extends Error 
  ? { error: true; message: string }
  : { error: false; data: T };

// Extract promise type
type UnwrapPromise<T> = T extends Promise<infer U> ? U : T;

// Deep readonly
type DeepReadonly<T> = {
  readonly [P in keyof T]: T[P] extends object ? DeepReadonly<T[P]> : T[P];
};

// Type-safe API routes
type HttpMethod = 'GET' | 'POST' | 'PUT' | 'DELETE';
type ApiRoute = `/api/${string}`;
type FullRoute = `${HttpMethod} ${ApiRoute}`;

// Type-safe CSS classes
type ComponentSize = 'sm' | 'md' | 'lg';
type ComponentVariant = 'primary' | 'secondary' | 'danger';
type ButtonClass = `btn-${ComponentSize}-${ComponentVariant}`;

// Branded types for type safety
type UserId = string & { readonly __brand: unique symbol };
type ProductId = string & { readonly __brand: unique symbol };

function createUserId(id: string): UserId {
  return id as UserId;
}

// Usage prevents mixing up IDs
function fetchUser(id: UserId): Promise<User> {
  // Type system prevents passing ProductId
}

interface UserRepository {
  findById(id: string): Promise<User | null>;
  save(user: User): Promise<void>;
}

interface EmailService {
  send(to: string, subject: string, body: string): Promise<void>;
}

class UserService {
  constructor(
    private readonly userRepo: UserRepository,
    private readonly emailService: EmailService
  ) {}

  async registerUser(email: string): Promise<User> {
    const user = await this.userRepo.save({ email });
    await this.emailService.send(email, 'Welcome', '...');
    return user;
  }
}

type Result<T, E = Error> = 
  | { success: true; data: T }
  | { success: false; error: E };

async function fetchUser(id: string): Promise<Result<User, 'NOT_FOUND' | 'NETWORK_ERROR'>> {
  try {
    const user = await api.get(`/users/${id}`);
    return { success: true, data: user };
  } catch (error) {
    return { success: false, error: 'NOT_FOUND' };
  }
}

// Usage forces explicit error handling
const result = await fetchUser('123');
if (result.success) {
  console.log(result.data.email);
} else {
  console.error(result.error);
}

interface Repository<T, ID = string> {
  findById(id: ID): Promise<T | null>;
  findAll(): Promise<T[]>;
  save(entity: T): Promise<T>;
  delete(id: ID): Promise<void>;
}

class InMemoryUserRepository implements Repository<User> {
  private users = new Map<string, User>();
  
  async findById(id: string): Promise<User | null> {
    return this.users.get(id) ?? null;
  }
  
  // ... other methods
}

// Easy to swap implementations
const userRepo: Repository<User> = new InMemoryUserRepository();
// Later: const userRepo = new PostgresUserRepository();

// Use tree-shaking friendly exports
export { specificFunction } from './utils';
// Avoid: export * from './utils';

// Lazy load heavy dependencies
const loadHeavyLibrary = async () => {
  const { HeavyLibrary } = await import('./heavy-library');
  return HeavyLibrary;
};

// Use const enums for compile-time inlining
const enum Status {
  Active = 1,
  Inactive = 2,
}

// Memoize expensive computations
function memoize<Args extends unknown[], Return>(
  fn: (...args: Args) => Return
): (...args: Args) => Return {
  const cache = new Map<string, Return>();
  return (...args: Args) => {
    const key = JSON.stringify(args);
    if (!cache.has(key)) {
      cache.set(key, fn(...args));
    }
    return cache.get(key)!;
  };
}

// Batch operations
async function batchProcess<T>(
  items: T[],
  batchSize: number,
  processor: (batch: T[]) => Promise<void>
): Promise<void> {
  for (let i = 0; i < items.length; i += batchSize) {
    await processor(items.slice(i, i + batchSize));
  }
}

class DomainError extends Error {
  constructor(
    message: string,
    public readonly code: string,
    public readonly statusCode: number = 500
  ) {
    super(message);
    this.name = this.constructor.name;
  }
}

class NotFoundError extends DomainError {
  constructor(resource: string) {
    super(`${resource} not found`, 'NOT_FOUND', 404);
  }
}

class ValidationError extends DomainError {
  constructor(message: string, public readonly fields: string[]) {
    super(message, 'VALIDATION_ERROR', 400);
  }
}

// Type-safe error handling
function handleError(error: unknown): DomainError {
  if (error instanceof DomainError) {
    return error;
  }
  return new DomainError('Internal server error', 'INTERNAL_ERROR');
}

async function fetchWithRetry<T>(
  fn: () => Promise<T>,
  maxRetries = 3,
  delay = 1000
): Promise<Result<T, Error>> {
  for (let attempt = 0; attempt < maxRetries; attempt++) {
    try {
      const data = await fn();
      return { success: true, data };
    } catch (error) {
      if (attempt === maxRetries - 1) {
        return { success: false, error: error as Error };
      }
      await new Promise(resolve => setTimeout(resolve, delay * (attempt + 1)));
    }
  }
  return { success: false, error: new Error('Max retries exceeded') };
}

// Separate pure functions from side effects
// Pure function - easy to test
function calculateTotal(items: LineItem[]): number {
  return items.reduce((sum, item) => sum + item.price * item.quantity, 0);
}

// Side effect - inject dependencies for testing
class OrderService {
  constructor(
    private readonly taxCalculator: TaxCalculator,
    private readonly logger: Logger
  ) {}
  
  async processOrder(order: Order): Promise<void> {
    const tax = this.taxCalculator.calculate(order);
    this.logger.info(`Processing order with tax: ${tax}`);
    // ... process order
  }
}

// Test utilities
export function createMockRepository<T>(
  initialData: T[] = []
): Repository<T> {
  const data = new Map<string, T>();
  initialData.forEach(item => {
    data.set((item as any).id, item);
  });
  
  return {
    findById: async (id: string) => data.get(id) ?? null,
    findAll: async () => Array.from(data.values()),
    save: async (entity: T) => {
      const id = (entity as any).id;
      data.set(id, entity);
      return entity;
    },
    delete: async (id: string) => {
      data.delete(id);
    },
  };
}

src/
├── features/
│   ├── users/
│   │   ├── user.entity.ts
│   │   ├── user.repository.ts
│   │   ├── user.service.ts
│   │   ├── user.controller.ts
│   │   └── index.ts
│   └── orders/
│       └── ...
├── shared/
│   ├── types/
│   ├── utils/
│   └── errors/
└── infrastructure/
    ├── database/
    └── http/

// features/users/index.ts
export { User } from './user.entity';
export { UserRepository } from './user.repository';
export { UserService } from './user.service';
export type { UserDto, CreateUserDto } from './user.types';

// Usage
import { UserService, User, CreateUserDto } from '@/features/users';

interface AppConfig {
  database: {
    host: string;
    port: number;
  };
  api: {
    baseUrl: string;
    timeout: number;
  };
}

function loadConfig(): AppConfig {
  const config = {
    database: {
      host: process.env.DB_HOST ?? (() => {
        throw new Error('DB_HOST is required');
      })(),
      port: Number(process.env.DB_PORT ?? 5432),
    },
    api: {
      baseUrl: process.env.API_BASE_URL!,
      timeout: Number(process.env.API_TIMEOUT ?? 5000),
    },
  };
  
  // Type system ensures all required fields are present
  return config satisfies AppConfig;
}

/**
 * Calculates the total price including tax.
 * 
 * @param items - Array of line items with price and quantity
 * @param taxRate - Tax rate as a decimal (e.g., 0.08 for 8%)
 * @returns Total price including tax
 * 
 * @example
 * ```ts
 * const total = calculateTotalWithTax(
 *   [{ price: 10, quantity: 2 }],
 *   0.08
 * ); // Returns 21.6
 * ```
 */
function calculateTotalWithTax(
  items: Array<{ price: number; quantity: number }>,
  taxRate: number
): number {
  const subtotal = items.reduce((sum, item) => sum + item.price * item.quantity, 0);
  return subtotal * (1 + taxRate);
}

npm install --save-dev @biomejs/biome

{
  "$schema": "https://biomejs.dev/schemas/1.9.4/schema.json",
  "organizeImports": {
    "enabled": true
  },
  "linter": {
    "enabled": true,
    "rules": {
      "recommended": true,
      "correctness": {
        "noUnusedVariables": "error",
        "useExhaustiveDependencies": "warn"
      },
      "style": {
        "useImportType": "error",
        "useConst": "error",
        "noParameterAssign": "error"
      },
      "suspicious": {
        "noExplicitAny": "error",
        "noArrayIndexKey": "warn",
        "noAssignInExpressions": "error"
      },
      "performance": {
        "noDelete": "error"
      },
      "complexity": {
        "noForEach": "off",
        "useSimplifiedLogicExpression": "warn"
      }
    }
  },
  "formatter": {
    "enabled": true,
    "formatWithErrors": false,
    "indentStyle": "space",
    "indentWidth": 2,
    "lineWidth": 100,
    "lineEnding": "lf",
    "ignore": ["node_modules", "dist", "build"]
  },
  "javascript": {
    "formatter": {
      "quoteStyle": "single",
      "jsxQuoteStyle": "double",
      "trailingCommas": "es5",
      "semicolons": "always",
      "arrowParentheses": "always"
    }
  },
  "overrides": [
    {
      "include": ["*.test.ts", "*.spec.ts"],
      "linter": {
        "rules": {
          "suspicious": {
            "noExplicitAny": "off"
          }
        }
      }
    }
  ]
}

{
  "scripts": {
    "lint": "biome lint .",
    "lint:fix": "biome lint --write .",
    "format": "biome format --write .",
    "check": "biome check .",
    "check:fix": "biome check --write ."
  }
}

// .vscode/settings.json
{
  "[typescript]": {
    "editor.defaultFormatter": "biomejs.biome",
    "editor.formatOnSave": true
  },
  "[typescriptreact]": {
    "editor.defaultFormatter": "biomejs.biome",
    "editor.formatOnSave": true
  },
  "editor.codeActionsOnSave": {
    "quickfix.biome": "explicit",
    "source.organizeImports.biome": "explicit"
  }
}