Typescript/ジェネリック

export default function genericsSample() {
  // ジェネリック型を使わない場合
  const stringReduce = (array: string[], initialValue: string): string => {
    let result = initialValue
    for (let i = 0; i < array.length; i++) {
      const item = array[i]
      result += item
    }
    return result
  }
  console.log('Generics basic sample 1:', stringReduce(['May ', 'the ', 'force ', 'be ', 'with ', 'you'], ''))

  const numberReduce = (array: number[], initialValue: number): number => {
    let result = initialValue
    for (let i = 0; i < array.length; i++) {
      const item = array[i]
      result += item
    }
    return result
  }
  console.log('Generics basic sample 2:', numberReduce([100, 200, 300], 1000))

  // 型エイリアスを作るとしたら...？
  type Reduce = {
    (array: string[], initialValue: string): string
    (array: number[], initialValue: number): number
  }

  // ジェネリック型でまとめる
  type GenericReduce = {
    (array: T[], initialValue: T): T
  }

  // ジェネリック型を使った呼び出し
  const genericStringReduce: GenericReduce = (array, initialValue) => {
    let result = initialValue
    for (let i = 0; i < array.length; i++) {
      const item = array[i]
      result += item
    }
    return result
  }
  console.log('Generics basic sample 3:', genericStringReduce(['MAKE ', 'TYPESCRIPT ', 'GREAT ', 'AGAIN'], ''))

  const genericNumberReduce: GenericReduce = (array, initialValue) => {
    let result = initialValue
    for (let i = 0; i < array.length; i++) {
      const item = array[i]
      result += item
    }
    return result
  }
  console.log('Generics basic sample 4:', numberReduce([-100, -200, -300], 1000))

  // いろいろなジェネリック型の定義方法
  // 完全な呼び出しシグネチャ（個々のシグネチャにジェネリック型を割り当てる）
  type GenericReduce2 = {
    (array: T[], initialValue: T): T
    (array: U[], initialValue: U): U
  }
  // 呼び出しシグネチャの省略記法
  type GenericReduce3 = (array: T[], initialValue: T) => T
  type GenericReduce4 = (array: T[], initialValue: T) => T
}