meh_programmer meh_programmer - 6 months ago 117
Node.js Question

Using async/await with a forEach loop

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

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 (let 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 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)
  }));
}
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