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Pipe Promise

Implement Promise Pipeline

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Promise Pipeline 實作

難度：Medium

題目描述

實作一個 pipePromise 函數，可以將多個回傳 Promise 的函數串接起來：
type FnPromise = (...args: any[]) => Promise<any>;

/**
 * Executes a series of promise-returning functions in a pipeline fashion.
 *  pipePromise(fn1, fn2, fn3, ...)
 *
 * @param fns - The promise-returning functions to be executed in order.
 * @returns A promise that resolves to the final result of the pipeline.
 * @example
 *    const add =  (a) => Promise.resolve(a + 1);
 *    const toString =  (a) => Promise.resolve(a + '');
 *    const format = pipePromise(add, toString);
 *    await format(1)  -->  '2'
 *    await format(9527)  -->  '9528'
 */
export function pipePromise<T extends FnPromise[]>(...fns: T) {
  // TODO Please add your codes here
}

// 範例
const add = a => Promise.resolve(a + 1);
const toString = a => Promise.resolve(`${a}`);

const format = pipePromise(add, toString);
await format(1); // → '2'
await format(9527); // → '9528'

實作要求

  1. 型別定義
    • 正確處理泛型型別
    • 保持型別推導
    • 支援任意數量的函數
  2. 錯誤處理
    • 正確傳遞錯誤
    • 保持 Promise 鏈
    • 清理資源
Promise
TypeScript
FP

解題思路

1. 基本實作

最簡單的實作方式是使用 reduce：

export function pipePromise<T extends FnPromise[]>(...fns: T) {
  return async (input: any) => {
    return fns.reduce(async (promise, fn) => {
      const result = await promise;
      return fn(result);
    }, Promise.resolve(input));
  };
}

2. 簡潔版本

避免不必要的 Promise 包裝：

export function pipePromise<T extends FnPromise[]>(...fns: T) {
  return async (input: any) => {
    let result = input;

    for (const fn of fns) {
      result = await fn(result);
    }

    return result;
  };
}

重要概念

1. Promise 串接

  • 每個函數都必須回傳 Promise
  • 前一個函數的結果會傳給下一個函數
  • 需要正確處理 async/await

2. 效能考量

  • 避免不必要的 Promise 包裝
  • 減少記憶體使用

測試範例

describe('pipePromise', () => {
  const add = (a: number) => Promise.resolve(a + 1);
  const multiply = (a: number) => Promise.resolve(a * 2);
  const toString = (a: number) => Promise.resolve(String(a));

  test('basic pipe', async () => {
    const pipe = pipePromise(add, toString);
    expect(await pipe(1)).toBe('2');
  });

  test('multiple functions', async () => {
    const pipe = pipePromise(add, multiply, toString);
    expect(await pipe(1)).toBe('4');
  });

  test('error handling', async () => {
    const throwError = () => Promise.reject(new Error('test'));
    const pipe = pipePromise(add, throwError, toString);
    await expect(pipe(1)).rejects.toThrow('test');
  });
});

進階討論

  1. 為什麼不用 Promise.then?
    • async/await 更現代化
    • 錯誤處理更直觀
  2. 實際應用場景
    • API 請求串接
    • 資料轉換流程
    • 非同步操作集合
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