javascript - 无法从 promise.all() 获得响应

Question

我发现使用promise.all时无法得到响应。

下面的orders应该是一个数组

Promise.all(promises).then(function (orders) {
  console.log(orders); // nothing logged
});

完整代码

let promises = markets.map(async (mkt) => {
  return new Promise(async function (resolve, reject) {
    return functionA(paramA)
      .then(resA => {
        return res.functionB()
          .then((resB) => {
            if (resB.length > 0) {
              return functionC()
                .then(resC => {
                  console.log(resC); // successfully logged
                  return resC;
                });
            }
          });
      })
      .catch(err => console.log(err));
  });
});
Promise.all(promises).then(function (orders) {
  console.log(orders); // nothing logged
});

我该如何解决？

更新 1
我根据评论更新了代码。
orders 现在正在返回

(54) [undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, Array(1), undefined, undefined, undefined, undefined, undefined, undefined, undefined, Array(1), undefined, Array(1), undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined]

如果没有undefined，我如何让承诺返回？

const promises = markets.map(async mkt => {
  const resA = await functionA(paramA);
  const resB = await res.functionB();
  if (resB.length > 0) {
    const resC = await functionC()
    console.log(resC); // successfully logged
    return resC;
  }
});
Promise.all(promises).then(orders => {
  console.log(orders); // nothing logged
}, err => {
  console.log(err);
});

Answer 1

• Promise 构造函数忽略从其回调返回的任何值
• 你没有在任何地方打电话给resolve
• 当你调用一个已经给你一个 Promise 的函数时，你不需要构造一个 Promise

const promises = markets.map((mkt) => {
    return functionA(paramA)
        .then(resA => res.functionB())
        .then((resB) => {
            if (resB.length > 0) {
                return functionC()
                    .then(resC => {
                        console.log(resC); // successfully logged
                        return resC;
                    });
            }
        })
        .catch(handleError);
});

更好的方法是利用await 使代码扁平化且易于阅读。

const promises = markets.map(async (mkt) => {
    try {
        const resA = await functionA(paramA);
        const resB = await res.functionB();

        if (resB.length > 0) {
            const resC = await functionC()
            return resC;
        }
    } catch (e) {
        // handle errors
    }
});

您还可以考虑在 Promise.all 中处理错误，而不是在映射器中。

如果您只想包含调用了functionC 的项目，请在之后过滤掉空值。

Promise.all(promises).then(function (orders) {
  const filtered = orders.filter(Boolean);
  console.log(filtered);
});

Answer 2

嗯，你从来没有 resolve() 或 reject() 你正在创建的 new Promises。避免使用Promise constructor antipattern 和never pass an async function as the executor to new Promise！

通过使用async/await 而不是.then()，可以大大简化此代码：

const promises = markets.map(async mkt => {
  const resA = await functionA(paramA);
  const resB = await res.functionB();
  if (resB.length > 0) {
    const resC = await functionC()
    console.log(resC); // successfully logged
    return resC;
  }
});
Promise.all(promises).then(orders => {
  console.log(orders); // nothing logged
}, err => {
  console.log(err);
});