回答

// 自定义 Promise.all 的实现
function myPromiseAll(promises) {
  return new Promise((resolve, reject) => {
    let results = [];
    let count = 0;

    for (let i = 0; i < promises.length; i++) {
      promises[i].then((result) => {
        results[i] = result;
        count++;

        if (count === promises.length) {
          resolve(results);
        }
      }).catch((error) => {
        reject(error);
      });
    }
  });
}

// 使用 Promise.all
let p1 = Promise.resolve(1);
let p2 = Promise.resolve(2);
let p3 = Promise.resolve(3);

Promise.all([p1, p2, p3]).then((results) => {
  console.log(results); // [1, 2, 3]
}).catch((error) => {
  console.error(error);
});

// 使用自定义 Promise.all
let p4 = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve(4);
  }, 1000);
});

let p5 = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve(5);
  }, 500);
});

myPromiseAll([p4, p5]).then((results) => {
  console.log(results); // [4, 5]
}).catch((error) => {
  console.error(error);
});