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");

• testing_22: That first await is not inside an async function, so it won't give the desired outcome
• Iceberg: You didn't use fs.readdir() correctly, and it should be const files = await readdir(path);

Answer 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");

Answer 2

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();

Answer 3

EDIT:

1. await cannot be used in a global context.
2. 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.
3. 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.

• Miracle Hades: As I checked, the fs.readdir() does not support .then()

Answer 4

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");
});

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