Nested async execution is not working properly

Question

I have a function will send all the images that located inside the "samples" folder to extractImage function, then, the extractImage function will call 3rd party API to perform its operation. But when I try to console.log(arr), it seem like didn't even be called. I think I have did something wrong to handling the async, could someone help me to have a look. I have quick new to JavaScript.

let arr = [];

await fs.readdir("samples", async (err, files) => {
  console.log(files);
  files.map(async (val) => {
    console.log(val);
    let tt = await extractImage(val);
    return arr.push(tt);
  });
});

fs.writeFileSync("final.json", "s");

console.log(arr);
console.log("tt");

That first await is not inside an async function, so it won't give the desired outcome — testing_22
– testing_22, Commented Sep 2, 2021 at 1:35
You didn't use fs.readdir() correctly, and it should be const files = await readdir(path); — Iceberg
– Iceberg, Commented Sep 2, 2021 at 2:02

Iceberg · Accepted Answer · 2021-09-02 02:25:07Z

1

You didn't use fs.readdir() correctly, and it should be const files = await readdir(path). And your files.map(async (val) => {...}) will return a list of promises immediately, and console.log(arr) might be executed even before these promises resolve. The code might be like the following one:


let files = await fs.readdir("samples")
console.log(files);

let promises = files.map(async (val) => {
    let tt = await extractImage(val);
    return tt;
});

let arr = await Promise.all(promises)
console.log(arr);
console.log("tt");

fs.writeFileSync("final.json", "s");

edited Sep 2, 2021 at 2:25

answered Sep 2, 2021 at 2:10

Iceberg

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Comments

Andy · Accepted Answer · 2021-09-02 02:06:12Z

1

I might consider using promisify in this instance, and then returning an array of promises from the files, and getting your data when all of those have been resolved.

const util = require('util');
const fs = require('fs');

const readdirP = util.promisify(fs.readdir);

async function main() {
  try {
    const files = await readdirP('samples');
    const promises = files.map(file => extractImage(file));
    const data = await Promise.all(promises);
  } catch(error) {
    console.log(error);
  }
}

main();

answered Sep 2, 2021 at 2:06

Andy

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Comments

Kenneth Lew · Accepted Answer · 2021-09-02 02:09:37Z

1

EDIT:

await cannot be used in a global context.
fs.readdir does not return a promise. It is callback based, which means you cannot await it. However, there is a synchronous version of the method: fs.readdirSync.
As of Node 11, you can do this:

const fs = require('fs').promises

which allows the use of await and .then on fs. For lower versions, you can use util.promisify.

edited Sep 2, 2021 at 2:09

answered Sep 2, 2021 at 1:40

Kenneth Lew

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1 Comment

Miracle Hades Over a year ago

As I checked, the fs.readdir() does not support .then()

Ryan Le · Accepted Answer · 2021-09-02 01:48:43Z

0

Using a self-invoked function could help in this case:

(async function () {
  let arr = [];
  await fs.readdir("samples", async (err, files) => {
    console.log(files);
    files.map(async (val) => {
      console.log(val);
      let tt = await extractImage(val);
      return arr.push(tt);
    });
  });
  fs.writeFileSync("final.json", "s");
  console.log(arr);
  console.log("tt");
});

answered Sep 2, 2021 at 1:48

Ryan Le

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1 Comment

Miracle Hades Over a year ago

I have tested this solution before but it seem like the console.log(arr); didn't wait for fs.readdir() to complete

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