TypeScriptで条件付き型をマスターする

1type IsString<T> = T extends string ? "It's a string" : "It's not a string";

1type Config = {
2  mode: 'simple' | 'detailed';
3};
4
5type Response<T extends Config> = T['mode'] extends 'simple'
6  ? { data: string }
7  : { data: string; details: string[] };
8
9function fetchData<T extends Config>(config: T): Response<T> {
10  if (config.mode === 'simple') {
11    return { data: 'Simplified data' } as Response<T>;
12  } else {
13    return {
14      data: 'Full data',
15      details: ['detail1', 'detail2'],
16    } as Response<T>;
17  }
18}
19
20const simpleConfig = { mode: 'simple' };
21const detailedConfig = { mode: 'detailed' };
22
23const resultSimple = fetchData(simpleConfig); // { data: "Simplified data" }
24const resultDetailed = fetchData(detailedConfig); // { data: "Full data", details: [...] }

1type UserResponse<
2  T extends Config,
3  Authenticated extends boolean,
4> = Authenticated extends true
5  ? T['mode'] extends 'simple'
6    ? { data: string }
7    : { data: string; details: string[] }
8  : { error: 'Not authenticated' };
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: 'Not authenticated' } as UserResponse<T, Authenticated>;
16  }
17  if (config.mode === 'simple') {
18    return { data: 'Simplified data' } as UserResponse<T, Authenticated>;
19  } else {
20    return {
21      data: 'Full data',
22      details: ['detail1', 'detail2'],
23    } as UserResponse<T, Authenticated>;
24  }
25}
26
27const resultAuthSimple = fetchUserData(simpleConfig, true); // { data: "Simplified data" }
28const resultAuthDetailed = fetchUserData(detailedConfig, true); // { data: "Full data", details: [...] }
29const resultNotAuth = fetchUserData(detailedConfig, false); // { error: "Not authenticated" }

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