TypeScript: MapをEnumよりも優先すべき理由

1// TypeScriptコード
2enum Status {
3  Active = 'ACTIVE',
4  Inactive = 'INACTIVE',
5}
6
7// トランスパイルされたJavaScriptコード
8var Status = {
9  Active: 'ACTIVE',
10  Inactive: 'INACTIVE',
11  ACTIVE: 'Active',
12  INACTIVE: 'Inactive',
13};
14(function (Status) {
15  Status['Active'] = 'ACTIVE';
16  Status['Inactive'] = 'INACTIVE';
17})(Status || (Status = {}));

1// constと型による解決策
2const Status = {
3  Active: 'ACTIVE',
4  Inactive: 'INACTIVE',
5} as const;
6
7// 自動的に推論される型
8type Status = (typeof Status)[keyof typeof Status];
9
10// シンプルで効率的なトランスパイル結果
11const Status = {
12  Active: 'ACTIVE',
13  Inactive: 'INACTIVE',
14};

1function processStatus(status: Status) {
2  // 無効なステータスでコンパイルエラー
3  console.log(status);
4}
5
6// ランタイム検証
7const isValidStatus = (status: string): status is Status =>
8  Object.values(Status).includes(status as Status);

1// 動的な追加が可能
2const ExtendedStatus = {
3  ...Status,
4  Pending: 'PENDING' as const,
5};

1// 関連設定
2const StatusConfig = {
3  [Status.Active]: {
4    color: 'green',
5    label: '有効',
6    icon: 'check',
7  },
8  [Status.Inactive]: {
9    color: 'red',
10    label: '無効',
11    icon: 'cross',
12  },
13} as const;
14
15// ユーティリティ型
16type StatusConfig = {
17  [K in Status]: {
18    color: string;
19    label: string;
20    icon: string;
21  };
22};
23
24// 型付きヘルパー関数
25function getStatusConfig(status: Status) {
26  return StatusConfig[status];
27}

1const ApiEndpoints = {
2  Users: '/api/users',
3  Products: '/api/products',
4  Orders: '/api/orders',
5} as const;
6
7type Endpoint = (typeof ApiEndpoints)[keyof typeof ApiEndpoints];
8
9// 自動的なURL型付け
10function fetchData(endpoint: Endpoint) {
11  return fetch(endpoint);
12}

1const LoadingState = {
2  Idle: 'IDLE',
3  Loading: 'LOADING',
4  Success: 'SUCCESS',
5  Error: 'ERROR',
6} as const;
7
8type LoadingState = typeof LoadingState[keyof typeof LoadingState];
9
10function handleState(state: LoadingState) {
11  switch (state) {
12    case LoadingState.Loading:
13      return <Spinner />;
14    case LoadingState.Error:
15      return <ErrorMessage />;
16    // TypeScriptが網羅性をチェック
17  }
18}

1// 推奨
2const HttpStatus = {
3  Ok: 200,
4  NotFound: 404,
5  ServerError: 500,
6} as const;
7
8// 避けるべき
9const Status = {
10  a: 200,
11  b: 404,
12  c: 500,
13} as const;

1// 型安全なユニオンの作成
2type HttpSuccessStatus = 200 | 201 | 204;
3type HttpErrorStatus = 400 | 401 | 404 | 500;
4type HttpStatus = HttpSuccessStatus | HttpErrorStatus;

1function isHttpSuccess(status: number): status is HttpSuccessStatus {
2  return [200, 201, 204].includes(status);
3}