Typescript最新機能キャッチアップ：Template Literal TypesやRecursive Typesの徹底解説

type Greeting = 'Hello';
type Audience = 'World' | 'TypeScript';
type Message = `${Greeting}, ${Audience}!`;
// Message type is: "Hello, World!" | "Hello, TypeScript!"

type HTTPMethod = 'GET' | 'POST' | 'PUT' | 'DELETE';
type APIEndpoint = `/api/v1/${string}`;

function fetchAPI<T extends APIEndpoint>(endpoint: T) {
  // API実装
}

// OK
fetchAPI('/api/v1/users');
// Error
fetchAPI('/v1/users'); // APIEndpoint型に適合しない

type FileSystemItem = {
  name: string;
  children?: FileSystemItem[];
};

const fileSystem: FileSystemItem = {
  name: 'root',
  children: [
    {
      name: 'documents',
      children: [
        { name: 'report.pdf' }
      ]
    },
    { name: 'photos' }
  ]
};

type JSONValue = 
  | string
  | number
  | boolean
  | null
  | JSONValue[]
  | { [key: string]: JSONValue };

const validJSON: JSONValue = {
  name: "John",
  age: 30,
  hobbies: ["reading", "coding"],
  address: {
    city: "Tokyo",
    country: "Japan"
  }
};

// 深さ制限の例
type FileSystemItem<D extends number = 3> = {
  name: string;
  children?: D extends 0 ? never : FileSystemItem<Decrement<D>>[];
};

type Decrement<N extends number> = N extends 0 ? 0 
  : N extends 1 ? 0 
  : N extends 2 ? 1 
  : N extends 3 ? 2 
  : never;