TypesScriptチップ:リターンオフオーバーロードに条件型を使用する.

導入

interface IRawUser {
  first_name: string
  email: string,
  id: number,
}

interface User {
  name: string,
  email: string
  print: () => void
}

declare function userFactory(rawUser: IRawUser): User;

function mapRawToUserObject(rawShow: IRawUser[]): User[];
function mapRawToUserObject(rawShow: IRawUser): User;
function mapRawToUserObject(rawShow: IRawUser | IRawUser[]): User | User[] {
  if (rawShow instanceof Array) {
    return rawShow.map((raw) => userFactory(raw));
  }

  return userFactory(rawShow);
}

条件型を作成する

type MapRawResult<T extends IRawUser | IRawUser[]> = any;

type MapRawResult<T extends IRawUser | IRawUser[]> = T extends IRawUser ? User : User[]

In plain words: If T is assignable to IRawUser return User otherwise return User[]

条件式をこのメソッドに適用します。

function mapRawToUserObject(rawShow: IRawUser[]): User[];
function mapRawToUserObject(rawShow: IRawUser): User;
function mapRawToUserObject(rawShow: IRawUser | IRawUser[]): User | User[] {
  // implementation
}

type MapRawResult<T extends IRawUser | IRawUser[]> = T extends IRawUser ? User : User[]

function mapRawToUserObject<T extends IRawUser | IRawUser[]>(rawShow: T): MapRawResult<T> {
  // implementation
}

function mapRawToUserObject<T extends IRawUser | IRawUser[]>(rawShow: T): MapRawResult<T> {
  if (rawShow instanceof Array) {
    return rawShow.map((raw) => userFactory(raw)) as MapRawResult<T>
  }

  return userFactory(rawShow) as MapRawResult<T>
}

結論

type MapRawArg = IRawUser | IRawUser[];

type MapRawResult<T extends MapRawArg> = T extends IRawUser ? User : User[];
function mapRawToUserObject<T extends MapRawArg>(rawShow: T): MapRawResult<T> {
  // ...
}

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