Originally published @ puruvj.dev

Let's consider the code below

const IDs = [1, 2, 3];

const usersData = IDs.map(async (id) => await getUserData(id));

console.log(usersData);

What would this output?

[Promise, Promise, Promise];

All these are promises. But we are expecting data as objects(or whatever other format you can think of) here. How do await every single array item?

The solution here is Promise.all. Quick recap:

Promise.all takes in an array of promises, runs them concurrently until they all resolve, and return a bigger promise with the outcomes from those promises as resolved values as an Array

For example

await Promise.all([getUserData(1), getUserData(2), getUserData(3)]);

will return

[
  { id: 1, ...otherData },
  { id: 2, ...otherData },
  { id: 3, ...otherData },
];

If you think about it, the code snippet where we're mapping over IDs is just an Array of Promises. We can directly Promise.all that array

const IDs = [1, 2, 3];

const usersDataPromises = IDs.map(async (id) => await getUserData(id));

const usersData = await Promise.all(usersDataPromises);

console.log(usersData);

That would output us the same object as above

[
  { id: 1, ...otherData },
  { id: 2, ...otherData },
  { id: 3, ...otherData },
];

Tricky part

The trick above works like a charm. However, difficulty arises when you chain another array method to the existing array, like this

const IDs = [1, 2, 3];

const usersDataPromise = IDs.map(async (id) => await getUserData(id)).map(
  async (data) => await getPosts(data)
);

const usersData = Promise.all(usersDataPromise);

console.log(usersData);

It will return an error. Why?

Promise.all tries to run all promises at once. And I mean, All of them. It will try to run the 2nd map alongside the first map. You can see for yourself this is a problem, as the second map depends on the value from the first.

How do we resolve this (Pun intended 😎)?

Solutions

There can be many ways to solve this problem. I will share 2 here

1st

Promise.all at every single step

const IDs = [1, 2, 3];

const usersData = await Promise.all(
  IDs.map(async (id) => await getUserData(id))
);

const usersPosts = await Promise.all(
  usersData.map(async (userData) => await getPosts(userData))
);

2nd

A plain old for of loop:

const IDs = [1, 2, 3];

const usersPosts = [];

for (let id of IDs) {
  const userData = await getUsersData(id);

  const userPosts = await getPosts(userData);

  usersPosts.push(userPosts);
}

I prefer the 2nd approach. If you wanna add an extra step, you simply add an extra line, whereas the 2st will require a whole extra Promise.all(array map), which ultimately is just code redundancy.