Using async/await with a forEach loop

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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.

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.

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);
  }
}

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)
  }));
}

JavaScript: async/await with forEach(), JavaScript: async/await with forEach(). Sebastien async function asyncForEach​(array, callback) { We now have a way of using forEach with async/await. Iterating Asynchronously: How to use async & await with foreach in C#. Well, there is it, that is how you get around the problem of calling async methods in a foreach loop. This article was

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

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

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

See spec: proposal-async-iteration


2018-09-10: This answer has been getting a lot attention recently, please see Axel Rauschmayer's blog post for further information about asynchronous iteration: ES2018: asynchronous iteration

The Pitfalls of Async/Await in Array Loops - DailyJS, Using async/await while looping through arrays in Javascript loop seems forEach . The first problem is that you can't await the entire loop when using forEach . Just use a modern for … of loop instead, in which await will work as expected: If you want to read the files in parallel, you cannot use forEach indeed. Each of the the async callback function calls does return a promise, but you're throwing them away instead of awaiting them.

Instead of Promise.all in conjunction with Array.prototype.map (which does not guarantee the order in which the Promises are resolved), I use Array.prototype.reduce, starting with a resolved Promise:

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

  await files.reduce(async (promise, file) => {
    // This line will wait for the last async function to finish.
    // The first iteration uses an already resolved Promise
    // so, it will immediately continue.
    await promise;
    const contents = await fs.readFile(file, 'utf8');
    console.log(contents);
  }, Promise.resolve());
}

JavaScript async and await in loops, Await in a forEach loop​​ (Notice the async keyword in the callback function. We need this async keyword because await is in the callback function). But the actual result is different. JavaScript proceeds to call console. JavaScript async and await in loops 1st May 2019. Basic async and await is simple. Things get a bit more complicated when you try to use await in loops.. In this article, I want to share some gotchas to watch out for if you intend to use await in loops.

The p-iteration module on npm implements the Array iteration methods so they can be used in a very straightforward way with async/await.

An example with your case:

const { forEach } = require('p-iteration');
const fs = require('fs-promise');

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

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

Why does async/await in a .forEach not actually await? – Corey Cleary, If you're trying to loop over a list while using async/await in Node.js (or the browser, for that matter), reaching for the .forEach array function might seem like a  async/await is freaking awesome, but there is one place where it’s tricky: inside a forEach () If you run this code with node.js (≥ 7.6.0), this will happen: console.log (num) are not displayed into the console. Let’s re-create forEach () to understand what’s happening: The real polyfill of forEach ()is available at https://developer

Here are some forEachAsync prototypes. Note you'll need to await them:

Array.prototype.forEachAsync = async function (fn) {
    for (let t of this) { await fn(t) }
}

Array.prototype.forEachAsyncParallel = async function (fn) {
    await Promise.all(this.map(fn));
}

Note while you may include this in your own code, you should not include this in libraries you distribute to others (to avoid polluting their globals).

JavaScript loops - how to handle async/await, How to run async loops in sequence or in parallel? I am using it more frequently now. forEach(async (item) => { await delayedLog(item); }  Also please note that, in case your methods are not by default Async, then you may plan to wrap them in a Task.Run, to await upon, though this would need a Thread pool thread, but can be called using Async-Await. Parallel.Foreach code for your use case (I am doing direct replacement, there seems to be an issue in your code, since ProcessCards

Breaking down confusion of combining Async/Await with Array , Using async/await in popular array methods (Array.forEach(), Array.map() etc.) can be quite confusing. Let's try to break it down and understand  The way you're using the await keyword tells C# that you want to wait each time you pass through the loop, which isn't parallel. You can rewrite your method like this to do what you want, by storing a list of Task s and then await ing them all with Task.WhenAll. public async Task<bool> Init() { var series = Enumerable.Range(1, 5).ToList(); var

Async/Await and the forEach Pit of Despair, Async/Await is like that same 4 year-old taking a nap. is all well and good, but what if we wanted to do an async call inside that forEach loop? in Async Await, C# When following the async await TAP pattern particularly when executing an async method inside a loop, such as a foreach delegate. Instead of writing a foreach loop you may want to make use of the List(T).ForEach extension that allows you to iterate over each item and perform an operation.

Using async/await with a forEach loop, Are there any issues with using asyncawait in a forEach loop Im trying to loop through an array of files and await on the contents of ea Questions: 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 =

Comments
  • Could you please explain why does for ... of ... work?
  • ok i know why... Using Babel will transform async/await to generator function and using forEach means that each iteration has an individual generator function, which has nothing to do with the others. so they will be executed independently and has no context of next() with others. Actually, a simple for() loop also works because the iterations are also in one single generator function.
  • @Demonbane: In short, because it was designed to work :-) await suspends the current function evaluation, including all control structures. Yes, it is quite similar to generators in that regard (which is why they are used to polyfill async/await).
  • @arve0 Not really, an async function is quite different from a Promise executor callback, but yes the map callback returns a promise in both cases.
  • When you come to learn about JS promises, but instead use half an hour translating latin. Hope you're proud @Bergi ;)
  • Upvoted, would be great if you could put a link to the spec in your answer for anyone who wants to know more about async iteration.
  • Shouldn't it be contents instead of file in the iterator
  • Why people are upvoting this answer? Take a closer look at the answer, question, and proposal. After the of should be the async function which will return an array. It doesn't work and Francisco said;
  • Totally agree with @AntonioVal. It's not an answer.
  • While I agree it's not an answer, upvoting a proposal is a way to increase its popularity potentially making it available earlier to use later on.
  • This works perfectly, thank you so much. Could you explain what is happening here with Promise.resolve() and await promise;?
  • This is pretty cool. Am I right in thinking the files will be read in order and not all at once?
  • @parrker9 Promise.resolve() returns an already resolved Promise object, so that reduce has a Promise to start with. await promise; will wait for the last Promise in the chain to resolve. @GollyJer The files will be processed sequentially, one at a time.
  • Very cool use of reduce, thanks for the comment! I'll just denote that, in contrast to some of the other methods mentioned in the comments, this one is synchronous, meaning that the files are read one after another and not in parallel (since the next iteration of reduce function relies on the previous iteration, it must be synchronous).