如何在循环内调用异步 function？

Question

I have below code in node. In getPosts , it reads 10 posts from database which is an async function call. And for each post, it needs to read user info. from database which is another async function call. How can I make it work in node js?

const getUser = async (userId) => {
    // read user from database
}

const getPosts =async () => {
  const posts = await getPostsFromDB(10); // get 10 posts from database
  for(let i=0; i<posts.length; i++){
        posts[i].user = await getUser(posts[i].userId) // ERROR: I can't call await inside a loop
  }

}

I am thinking about using Promise.all() like below:

const getPosts =async () => {
  const posts = await getPostsFromDB(10); // get 10 posts from database
  const allProms = posts.map(post => getUser(post.userId));
  Promise.all(allProms); // how can I assign each user to each post?

}

but I don't know how I can assign each user to each post after calling Promise.all() .

Answer 1

Consider approaching the problem slightly differently. If you wait for responses in an iterative loop, it'll produce poor performance. Instead, you could push them all into an array and wait for them — so they're all fetching at the same time.

const getUser = async (userId) => {
  try {
    // read
  } catch (e) {
    // catch errors
  }

  // return data
}

const getPosts = async () => {
  const posts = await getPostsFromDB(10); // get 10 posts from database
  const userRequests = posts.map((post, index) => getUser(post.userId))
  const users = await Promise.all(userRequests)

  return posts.map((post, index) => {
    post.user = users[index]
  })
}

If you think you may have duplicate userId s, consider forming a list of users you can reference before calling getUser .