typescript-types

name	typescript-types
description	Define TypeScript types, interfaces, and type utilities. Use when creating type definitions, DTOs, API response types, or component props.

This skill guides creation of TypeScript types and interfaces for Next.js applications.

File Location

• Types directory: src/types/ or src/types/index.ts
• Co-located types: In same file if only used locally
• Barrel exports: Use index.ts to re-export all types

Naming Conventions

Type	Convention	Example
Entity interfaces	PascalCase	Book, User, Order
Display/Card types	Entity + Data/Card suffix	BookCardData, UserProfile
API response types	Action + Response	LoginResponse, SearchResult
Props types	ComponentName + Props	BookCardProps, HeaderProps
Enums	PascalCase	OrderStatus, UserRole

Common Patterns

Entity Types (mirror Prisma models)

// src/types/index.ts
export interface Book {
  id: string;
  title: string;
  author: string;
  price: number;
  stock: number;
  coverUrl?: string;
  createdAt: Date;
  updatedAt: Date;
}

export interface Category {
  id: string;
  name: string;
  slug: string;
  parentId?: string;
}

Display/Card Types (for UI components)

// Subset of entity for display purposes
export interface BookCardData {
  id: string;
  title: string;
  author: string;
  price: number;
  coverUrl?: string;
  rating?: number;
  discountPercent?: number;
}

export interface CategoryWithCount {
  id: string;
  name: string;
  slug: string;
  bookCount: number;
  coverImage?: string;
}

API Response Types

// Generic API response wrapper
export interface ApiResponse<T> {
  success: boolean;
  data?: T;
  error?: {
    code: string;
    message: string;
  };
}

// Paginated response
export interface PaginatedResponse<T> {
  items: T[];
  total: number;
  page: number;
  pageSize: number;
  totalPages: number;
}

// Specific responses
export interface SearchBooksResponse {
  books: BookCardData[];
  total: number;
  query: string;
}

Component Props Types

// Props with children
export interface CardProps {
  children: React.ReactNode;
  className?: string;
}

// Props with data
export interface BookCardProps {
  book: BookCardData;
  onAddToCart?: (id: string) => void;
  showRating?: boolean;
}

// Props with handlers
export interface SearchBarProps {
  placeholder?: string;
  onSearch: (query: string) => void;
  onClear?: () => void;
}

Enums and Union Types

// Use const objects for better tree-shaking
export const OrderStatus = {
  PENDING: 'pending',
  CONFIRMED: 'confirmed',
  SHIPPED: 'shipped',
  DELIVERED: 'delivered',
  CANCELLED: 'cancelled',
} as const;

export type OrderStatus = typeof OrderStatus[keyof typeof OrderStatus];

// Simple union types
export type SortOrder = 'asc' | 'desc';
export type BookSortBy = 'price' | 'title' | 'createdAt' | 'rating';

Conversion Functions

// Entity to display type conversion
export function toBookCardData(book: Book): BookCardData {
  return {
    id: book.id,
    title: book.title,
    author: book.author,
    price: book.price,
    coverUrl: book.coverUrl,
  };
}

// Batch conversion
export function toBooksCardData(books: Book[]): BookCardData[] {
  return books.map(toBookCardData);
}

Best Practices

1. Export all types from src/types/index.ts for easy imports
2. Use interface for objects, type for unions/primitives
3. Make optional fields explicit with ? suffix
4. Avoid any - use unknown if type is truly unknown
5. Document complex types with JSDoc comments
6. Keep types close to usage if only used in one file