双语面试：实现一个 promiseAll 函数

面试题 Interview Question

实现一个 promiseAll 函数，行为与原生 Promise.all 一致。
Implement a promiseAll function that behaves like the native Promise.all.

要求 Requirements

1. promiseAll(promises) 接收一个由 Promise 或普通值组成的可迭代对象（如数组）
   promiseAll(promises) takes an iterable (e.g., an array) of values, which may be Promises or regular values.

2. 返回一个新的 Promise，满足以下条件：
   It returns a new Promise that:

   • 当所有 Promise 都成功时，按顺序返回它们的结果数组
     Resolves with an array of results in the same order as input, when all Promises resolve
   • 如果有任何一个 Promise 失败，则整个 Promise 立即失败
     Rejects immediately if any input Promise rejects

3. 不使用内置的 Promise.all
   Do not use the built-in Promise.all

4. 代码应遵循纯函数式风格，不使用 this
   Use pure functional style; do not rely on this

参考答案 Reference Solution

function promiseAll(iterable) {
  return new Promise((resolve, reject) => {
    const results = [];
    let completed = 0;
    const total = iterable.length;

    if (total === 0) return resolve([]);

    iterable.forEach((item, index) => {
      Promise.resolve(item)
        .then(value => {
          results[index] = value;
          completed++;
          if (completed === total) {
            resolve(results);
          }
        })
        .catch(reject); // 一旦有失败，直接 reject
    });
  });
}

示例 Example

const p1 = Promise.resolve(1);
const p2 = 42;
const p3 = new Promise(resolve => setTimeout(() => resolve('done'), 100));

promiseAll([p1, p2, p3]).then(result => {
  console.log(result); // 输出 Output: [1, 42, 'done']
});

promiseAll([Promise.resolve(1), Promise.reject('error')])
  .then(console.log)
  .catch(err => console.error('失败了 Failed:', err));
// 输出：失败了 Failed: error

面试考察点 Interview Focus

• 对 Promise 并发控制机制的理解
  Understanding of how Promises handle concurrency and resolution
• 异步流程控制能力（顺序控制 vs. 并发执行）
  Ability to manage asynchronous flow and order guarantees
• 错误处理与边界情况处理能力
  Ability to handle errors and edge cases robustly