内置工具类型

TypeScript 标准库提供大量 Utility Types，从已有类型派生新类型而无需重复字段。本课在 typescript-essentials 入门基础上系统讲解 Partial、Required、Pick、Omit、Record、Exclude、Extract、NonNullable、ReturnType、Parameters、Awaited 等，并展示在 nestjs-essentials DTO 与 nextjs-essentials 组件 props 中的典型用法。

学习目标

• 熟练使用 Pick/Omit/Partial/Required 组合
• 用 Exclude/Extract 操作联合类型
• 从函数签名提取 ReturnType 与 Parameters

对象变换

interface Lesson {
  id: number;
  slug: string;
  title: string;
  contentMd: string;
  publishedAt: Date | null;
}

type LessonCreate = Omit<Lesson, "id" | "publishedAt">;
type LessonUpdate = Partial<Pick<Lesson, "title" | "contentMd">>;
type LessonSummary = Pick<Lesson, "slug" | "title">;
type LessonEntity = Required<Pick<Lesson, "id" | "slug">>;

Record 与 Readonly

type TrackLevel = "beginner" | "intermediate" | "advanced";

type LevelLabels = Record<TrackLevel, string>;

const labels: LevelLabels = {
  beginner: "入门",
  intermediate: "进阶",
  advanced: "高级",
};

type FrozenConfig = Readonly<{ apiBase: string; timeout: number }>;

联合操作

type Status = "draft" | "published" | "archived";
type ActiveStatus = Exclude<Status, "archived">; // "draft" | "published"
type StringOrNumber = string | number | boolean;
type OnlyString = Extract<StringOrNumber, string>; // string

type MaybeId = number | null | undefined;
type DefiniteId = NonNullable<MaybeId>; // number

函数相关

async function fetchLesson(slug: string): Promise<Lesson> {
  const res = await fetch(`/api/lessons/${slug}`);
  return res.json();
}

type FetchLessonParams = Parameters<typeof fetchLesson>; // [slug: string]
type FetchLessonResult = ReturnType<typeof fetchLesson>; // Promise<Lesson>
type LessonData = Awaited<ReturnType<typeof fetchLesson>>; // Lesson

组合实战

type CreateLessonInput = Omit<Lesson, "id" | "publishedAt">;
type PatchLessonInput = Partial<Pick<Lesson, "title" | "contentMd" | "publishedAt">>;

function updateLesson(id: number, patch: PatchLessonInput): Promise<Lesson> {
  return fetch(`/api/lessons/${id}`, {
    method: "PATCH",
    body: JSON.stringify(patch),
  }).then((r) => r.json());
}

练习

• 从 platform 的 Track 类型派生 CreateTrackInput 与 TrackListItem
• 用 ReturnType 提取某 fetch 函数的 resolved 类型
• 用 Exclude 从联合中移除 "loading" 状态

小结

工具类型 = 类型层面的 DRY，与 REST PATCH/列表 DTO 模式天然契合。 下一课高级类型。