Promise.all with Async/Await

Let's say I have an API call that returns all the users from a database and takes some amount of time to complete.

// First promise returns an array after a delay
const getUsers = () => {
  return new Promise((resolve, reject) => {
    return setTimeout(
      () => resolve([{ id: 'jon' }, { id: 'andrey' }, { id: 'tania' }]),
      600
    )
  })
}

Now there's another call that relies on some information that exists in the entire array of users.

// Second promise relies on the result of first promise
const getIdFromUser = (user) => {
  return new Promise((resolve, reject) => {
    return setTimeout(() => resolve(user.id), 500)
  })
}

And a third call that modifies the second call.

// Third promise relies on the result of the second promise
const capitalizeIds = (id) => {
  return new Promise((resolve, reject) => {
    return setTimeout(() => resolve(id.toUpperCase()), 200)
  })
}

I might try to run the first call, then use a for...of loop to run the subsequent calls that rely on it.

const runAsyncFunctions = async () => {
  const users = await getUsers()

  for (let user of users) {
    const userId = await getIdFromUser(user)
    console.log(userId)

    const capitalizedId = await capitalizeIds(userId)
    console.log(capitalizedId)
  }

  console.log(users)
}

runAsyncFunctions()

However, this will be my output:

jon
JON
andrey
ANDREY
tania
TANIA
(3) [{…}, {…}, {…}]

I can use Promise.all() instead to run all of the first, then all the second, then all the third functions.

const runAsyncFunctions = async () => {
  const users = await getUsers()

  Promise.all(
    users.map(async (user) => {
      const userId = await getIdFromUser(user)
      console.log(userId)

      const capitalizedId = await capitalizeIds(userId)
      console.log(capitalizedId)
    })
  )

  console.log(users)
}

(3) [{…}, {…}, {…}]
jon
andrey
tania
JON
ANDREY
TANIA

Hope that helps someone.

Here's the whole snippet you can run in the console.

// First promise returns an array
const getUsers = () => {
  return new Promise((resolve, reject) => {
    return setTimeout(
      () => resolve([{ id: 'jon' }, { id: 'andrey' }, { id: 'tania' }]),
      600
    )
  })
}

// Second promise relies on the resulting array of first promise
const getIdFromUser = (user) => {
  return new Promise((resolve, reject) => {
    return setTimeout(() => resolve(user.id), 500)
  })
}

// Third promise relies on the result of the second promise
const capitalizeIds = (id) => {
  return new Promise((resolve, reject) => {
    return setTimeout(() => resolve(id.toUpperCase()), 200)
  })
}

const runAsyncFunctions = async () => {
  const users = await getUsers()

  Promise.all(
    users.map(async (user) => {
      const userId = await getIdFromUser(user)
      console.log(userId)

      const capitalizedId = await capitalizeIds(userId)
      console.log(capitalizedId)
    })
  )

  console.log(users)
}

runAsyncFunctions()

Promise.all with Async/Await

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