How to set variable = a value from a function result inside async function

Question

Inside a function, I would like to set the value of a variable (foldersInDir) to the results of getting the contents of a directory using fs.readdir();

I thought using await would force the console.log line to wait for a response, but it's not.

How can I set foldersInDir = the return value?

/*Begin function*/
const listContents = async (myPath) => {

    var fs = require('fs');

    let foldersInDir = await fs.readdir(myPath, function(err, items) {
        console.log(items); //works
        return  items;
    });

    console.log(foldersInDir); //does not work, undefined

}

Possible duplicate of [How do I return the response from an asynchronous call?](https://stackoverflow.com/questions/14220321/how-do-i-return-the-response-from-an-asynchronous-call) — Heretic Monkey, Oct 09 '18 at 23:48
Please check link : https://dev.to/damcosset/asynchronous-code-with-asyncawait-7cd. its help you — IftekharDani, Oct 10 '18 at 06:22

score 1 · Answer 1 · answered Oct 09 '18 at 23:56

1

You need to convert readdir to a promise, e.g.:

const foldersPromised = (path) =>
  new Promise((resolve, reject) =>
    fs.readdir(path, (err, items) =>
      err !== undefined ? reject(err) : resolve(items)
    )
  );
try {
  let foldersInDir = await foldersPromised(myPath);
} catch(err) {
  console.log(err);
}

answered Oct 09 '18 at 23:56

bredikhin

8,875
3
40
44

3

Just use [`fsPromises.readdir`](https://nodejs.org/api/fs.html#fs_fspromises_readdir_path_options) – Felix Kling Oct 09 '18 at 23:57
1

Good point, turns out it's already there starting with v10. Thanks for the heads-up! – bredikhin Oct 10 '18 at 00:01
@bredikhin - I get null when I try to console out foldersInDir – Tf11 Oct 10 '18 at 00:36
@Tf11 Hmm, maybe a scope issue? As per the listing `foldersInDir` will only exist within the `try` block. – bredikhin Oct 10 '18 at 01:15
@FelixKling unfortunately we are running v8 on prod now. – Tf11 Oct 10 '18 at 11:56

score 1 · Answer 2 · answered Oct 10 '18 at 02:31

1

const fs = require('fs');

const test = () => {
        let folders = fs.readdirSync('.');
        return folders;
}

console.log(test());

answered Oct 10 '18 at 02:31

Danilo Lemes

2,342
1
14
16

risingfire · Answer 3 · 2018-10-10T11:29:13.033

1

Edit: sorry, need to promisify() the function

const fs = require('fs');
const { promisify } = require('util') // available in node v8 onwards
const readdir = promisify(fs.readdir)

async function listContents() {
  try {                                         // wrap in try-catch in lieu of .then().catch() syntax
    const foldersInDir = await readdir(myPath)  // call promised function
    console.log('OK, folders:', foldersInDir)   // success
  } catch (e) {
    console.log('FAIL reading dir:', e)         // fail
  }  
}

listContents('path/to/folder') // run test

edited Oct 10 '18 at 11:29

answered Oct 10 '18 at 02:35

risingfire

19
4

So, `fs.readdir` returns a promise when no callback is passed? Can you point to some documentation where this is described? – Felix Kling Oct 10 '18 at 07:14

score 0 · Answer 4 · answered Oct 10 '18 at 02:25

0

I recommend using the promisify function provided by Node.js to fix the problem. This function will convert a callback-based function to a promise-based function, which can then be used using the await keyword.

const fs = require('fs');
const {
  promisify
} = require('util');

const readdirAsync = promisify(fs.readdir);

/*Begin function*/
const listContents = async(myPath) => {
  let foldersInDir = await readdirAsync(myPath);

  console.log(foldersInDir);
}

answered Oct 10 '18 at 02:25

Behrooz

2,181
1
16
24

[Node directly provides promise APIs for fs](https://nodejs.org/api/fs.html#fs_fspromises_readdir_path_options) – Felix Kling Oct 10 '18 at 07:13
@FelixKling I agree that it’s an alternative, but to be fair, it’s an experimental API added in the latest version of Node.js. For now, promisify seems to be more stable. – Behrooz Oct 10 '18 at 07:19

How to set variable = a value from a function result inside async function

4 Answers4