高级类型

映射类型、条件类型、字面量类型、模板字面量类型与递归类型是 TypeScript 类型系统的「元编程」层。本课深入这些机制，帮你读懂 nestjs-essentials 与 nextjs-essentials 依赖库中的复杂泛型，并在 typescript-essentials 泛型基础上构建可复用的类型工具。

学习目标

• 编写 mapped type 批量变换对象属性
• 使用 conditional type 与 infer 提取类型
• 用 template literal type 生成字符串联合

映射类型 Mapped Types

type Optional<T> = {
  [K in keyof T]?: T[K];
};

type Nullable<T> = {
  [K in keyof T]: T[K] | null;
};

type ReadonlyDeep<T> = {
  readonly [K in keyof T]: T[K] extends object ? ReadonlyDeep<T[K]> : T[K];
};

interface User {
  id: number;
  profile: { name: string };
}

type PartialUser = Optional<User>;

条件类型 Conditional Types

type IsArray<T> = T extends unknown[] ? true : false;

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

type ApiResult<T> = T extends { data: infer D } ? D : never;

async function load(): Promise<{ data: Lesson[] }> {
  return { data: [] };
}

type Lessons = UnwrapPromise<ReturnType<typeof load>>; // { data: Lesson[] }
type LessonList = ApiResult<Awaited<ReturnType<typeof load>>>; // Lesson[]

模板字面量类型

type HttpMethod = "GET" | "POST" | "PATCH" | "DELETE";
type ApiPath = `/api/${string}`;

type RouteKey = `${HttpMethod} ${ApiPath}`;

const routes: Record<RouteKey, () => void> = {
  "GET /api/tracks": () => {},
  "POST /api/lessons": () => {},
};

// 事件名模式
type EventName = `on${Capitalize<string>}`;

递归类型

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

type DeepPartial<T> = {
  [K in keyof T]?: T[K] extends object ? DeepPartial<T[K]> : T[K];
};

分布式条件类型

type ToArray<T> = T extends unknown ? T[] : never;
type StrOrNumArray = ToArray<string | number>; // string[] | number[]

type NonNullableFields<T> = {
  [K in keyof T]: NonNullable<T[K]>;
};

练习

• 实现 Mutable<T> 移除 readonly