掌握 TypeScript 条件类型

1type IsString<T> = T extends string ? '这是字符串' : '这不是字符串';

1type Config = {
2  mode: '简单' | '详细';
3};
4
5type Response<T extends Config> = T['mode'] extends '简单'
6  ? { data: string }
7  : { data: string; details: string[] };
8
9function fetchData<T extends Config>(config: T): Response<T> {
10  if (config.mode === '简单') {
11    return { data: '简化数据' } as Response<T>;
12  } else {
13    return {
14      data: '完整数据',
15      details: ['详情1', '详情2'],
16    } as Response<T>;
17  }
18}
19
20const simpleConfig = { mode: '简单' };
21const detailedConfig = { mode: '详细' };
22
23const resultSimple = fetchData(simpleConfig); // { data: "简化数据" }
24const resultDetailed = fetchData(detailedConfig); // { data: "完整数据", details: [...] }

1type UserResponse<
2  T extends Config,
3  Authenticated extends boolean,
4> = Authenticated extends true
5  ? T['mode'] extends '简单'
6    ? { data: string }
7    : { data: string; details: string[] }
8  : { error: '未认证' };
9
10function fetchUserData<T extends Config, Authenticated extends boolean>(
11  config: T,
12  isAuthenticated: Authenticated,
13): UserResponse<T, Authenticated> {
14  if (!isAuthenticated) {
15    return { error: '未认证' } as UserResponse<T, Authenticated>;
16  }
17  if (config.mode === '简单') {
18    return { data: '简化数据' } as UserResponse<T, Authenticated>;
19  } else {
20    return {
21      data: '完整数据',
22      details: ['详情1', '详情2'],
23    } as UserResponse<T, Authenticated>;
24  }
25}
26
27const resultAuthSimple = fetchUserData(simpleConfig, true); // { data: "简化数据" }
28const resultAuthDetailed = fetchUserData(detailedConfig, true); // { data: "完整数据", details: [...] }
29const resultNotAuth = fetchUserData(detailedConfig, false); // { error: "未认证" }

1type ReturnTypeOfFunction<T> = T extends (...args: any[]) => infer R
2  ? R
3  : never;
4
5function getHello(): string {
6  return '你好，世界！';
7}
8
9type HelloReturnType = ReturnTypeOfFunction<typeof getHello>; // 结果：string