TypeScript高级编程 typescript-advanced

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名称: typescript-advanced 描述: 包括泛型、条件类型、映射类型、模板字面量和类型守卫的高级TypeScript模式

TypeScript高级

带约束的泛型

interface HasId {
  id: string;
}

function findById<T extends HasId>(items: T[], id: string): T | undefined {
  return items.find(item => item.id === id);
}

function groupBy<T, K extends string | number>(
  items: T[],
  keyFn: (item: T) => K
): Record<K, T[]> {
  return items.reduce((acc, item) => {
    const key = keyFn(item);
    (acc[key] ??= []).push(item);
    return acc;
  }, {} as Record<K, T[]>);
}

type ApiResponse<T> = {
  data: T;
  meta: { page: number; total: number };
};

async function fetchApi<T>(url: string): Promise<ApiResponse<T>> {
  const res = await fetch(url);
  return res.json();
}

条件类型

type IsString<T> = T extends string ? true : false;

type Flatten<T> = T extends Array<infer U> ? U : T;

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

type FunctionReturn<T> = T extends (...args: any[]) => infer R ? R : never;

type ExtractRouteParams<T extends string> =
  T extends `${string}:${infer Param}/${infer Rest}`
    ? Param | ExtractRouteParams<Rest>
    : T extends `${string}:${infer Param}`
      ? Param
      : never;

type Params = ExtractRouteParams<"/users/:userId/posts/:postId">;

映射类型

type Readonly<T> = { readonly [K in keyof T]: T[K] };

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

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

type PickByType<T, V> = {
  [K in keyof T as T[K] extends V ? K : never]: T[K];
};

interface User {
  id: string;
  name: string;
  age: number;
  active: boolean;
}

type StringFields = PickByType<User, string>;

type EventMap<T> = {
  [K in keyof T as `on${Capitalize<string & K>}`]: (value: T[K]) => void;
};

type UserEvents = EventMap<{ login: User; logout: string }>;

可辨识联合类型

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

function divide(a: number, b: number): Result<number, string> {
  if (b === 0) return { ok: false, error: "Division by zero" };
  return { ok: true, value: a / b };
}

const result = divide(10, 2);
if (result.ok) {
  console.log(result.value);
} else {
  console.error(result.error);
}

type Shape =
  | { kind: "circle"; radius: number }
  | { kind: "rect"; width: number; height: number }
  | { kind: "triangle"; base: number; height: number };

function area(shape: Shape): number {
  switch (shape.kind) {
    case "circle": return Math.PI * shape.radius ** 2;
    case "rect": return shape.width * shape.height;
    case "triangle": return 0.5 * shape.base * shape.height;
  }
}

类型守卫

function isNonNull<T>(value: T | null | undefined): value is T {
  return value != null;
}

function hasProperty<K extends string>(
  obj: unknown,
  key: K
): obj is Record<K, unknown> {
  return typeof obj === "object" && obj !== null && key in obj;
}

const values = [1, null, 2, undefined, 3].filter(isNonNull);

function assertNever(value: never): never {
  throw new Error(`Unexpected value: ${value}`);
}

实用类型组合

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

type StrictOmit<T, K extends keyof T> = Pick<T, Exclude<keyof T, K>>;

type RequireAtLeastOne<T, Keys extends keyof T = keyof T> = Pick<T, Exclude<keyof T, Keys>> &
  { [K in Keys]-?: Required<Pick<T, K>> & Partial<Pick<T, Exclude<Keys, K>>> }[Keys];

type UpdateUserInput = RequireAtLeastOne<{ name: string; email: string; age: number }>;

反模式

• 使用 any 而不是 unknown 来处理类型不确定的值
• 使用类型断言 (as) 时，类型守卫会更安全
• 过于复杂的泛型签名会降低可读性
• 未使用可辨识联合类型来处理状态机或结果类型
• 当字符串字面量联合类型足够时使用 enum
• 在 tsconfig 中忽略 strictNullChecks

检查清单

• [ ] 在 tsconfig 中启用 strict: true
• [ ] 使用 unknown 而不是 any 处理外部数据
• [ ] 使用类型守卫安全地验证运行时类型
• [ ] 使用可辨识联合类型建模状态，并配合详尽switch语句
• [ ] 使用泛型约束 (extends) 防止误用
• [ ] 使用映射类型从源类型派生相关类型
• [ ] 优先使用实用类型（如 Pick、Omit、Partial），避免手动重打类型
• [ ] 使用模板字面量类型来强制字符串模式