Using async/await with a forEach loop

Question

Are there any issues with using async/await in a forEach loop? I'm trying to loop through an array of files and await on the contents of each file.

import fs from 'fs-promise'

async function printFiles () {
  const files = await getFilePaths() // Assume this works fine

  files.forEach(async (file) => {
    const contents = await fs.readFile(file, 'utf8')
    console.log(contents)
  })
}

printFiles()

This code does work, but could something go wrong with this? I had someone tell me that you're not supposed to use async/await in a higher-order function like this, so I just wanted to ask if there was any issue with this.

Answer 1

Sure the code does work, but I'm pretty sure it doesn't do what you expect it to do. It just fires off multiple asynchronous calls, but the printFiles function does immediately return after that.

Reading in sequence

If you want to read the files in sequence, you cannot use forEach indeed. Just use a modern for … of loop instead, in which await will work as expected:

async function printFiles () {
  const files = await getFilePaths();

  for (const file of files) {
    const contents = await fs.readFile(file, 'utf8');
    console.log(contents);
  }
}

Reading in parallel

If you want to read the files in parallel, you cannot use forEach indeed. Each of the async callback function calls does return a promise, but you're throwing them away instead of awaiting them. Just use map instead, and you can await the array of promises that you'll get with Promise.all:

async function printFiles () {
  const files = await getFilePaths();

  await Promise.all(files.map(async (file) => {
    const contents = await fs.readFile(file, 'utf8')
    console.log(contents)
  }));
}

Answer 2

With ES2018, you are able to greatly simplify all of the above answers to:

async function printFiles () {
  const files = await getFilePaths()

  for await (const contents of files.map(file => fs.readFile(file, 'utf8'))) {
    console.log(contents)
  }
}

See spec: proposal-async-iteration

Simplified:

  for await (const results of array) {
    await longRunningTask()
  }
  console.log('I will wait')

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2018-09-10: This answer has been getting a lot of attention recently, please see Axel Rauschmayer's blog post for further information about asynchronous iteration.