为什么我的函数返回 Promise { <pending>} [英] Why is my function returning Promise { <pending> }

问题描述

作为我的第一个真正的 MERN 项目，我正在构建一个留言板.我目前正在使用节点路由来请求板名称及其相关的帖子数，但我遇到了一个问题.我没有得到我需要的值，而是收到信息告诉我有一个未决的承诺，这在我使用 async/await 时看起来很奇怪.这是函数:

As my first real MERN project, I'm building a message board. I'm currently working on a node route to request the board names with their associated post count but I've hit an issue. Instead of getting the values that I need, I'm recieving info telling me there is a promise pending, which seems odd as I'm using async/await. Here's the function:

exports.postsPerBoard = async (req, res) => {
  try {
    const boards = await Board.find();

    const postCount = boards.map(async (boardI) => {
      const posts = await Post.find({ board: boardI.slug });
      return [boardI.slug, posts.length];
    });
    console.log(postCount);
    res.send(postCount);
  } catch (err) {
    console.error(err.message);
    res.status(500).send('server error');
  }
};

这是控制台日志的结果:

and here is the result of the console log:

[0] [
[0]   Promise { <pending> },
[0]   Promise { <pending> },
[0]   Promise { <pending> },
[0]   Promise { <pending> },
[0]   Promise { <pending> }
[0] ]

在此先感谢所有/任何帮助！:)

Thanks in advance for all/any help with this! :)

推荐答案

const postCount = boards.map(async (boardI) => {
  const posts = await Post.find({ board: boardI.slug });
  return [boardI.slug, posts.length];
});

由于这是一个异步函数，它将返回一个承诺.map 为数组的每个元素调用该函数，获取它们返回的承诺，并创建一个包含这些承诺的新数组.

Since this is an async function, it will return a promise. map calls the function for each element of the array, gets the promises they return, and creates a new array containing those promises.

如果您想等待每个完成的承诺数组，请使用 Promise.all 将它们组合成一个单一的承诺，然后等待结果.

If you would like to wait for that array of promises to each finish, use Promise.all to combine them into a single promise, and then await the result.

const promises = boards.map(async (boardI) => {
  const posts = await Post.find({ board: boardI.slug });
  return [boardI.slug, posts.length];
});
const postCount = await Promise.all(promises);
console.log(postCount);

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