fs.writeFile in a promise, asynchronous-synchronous stuff

I need some help with my code. I'm new at Node.js and have a lot of trouble with it.

What I'm trying to do:

1) Fetch a .txt with Amazon products (ASINs) ;

2) Fetch all products using the amazon-product-api package;

3) Save each product in a .json file.

My code is not working. I think I messed up with this asynchronous-synchronous stuff - help me!

var amazon = require('amazon-product-api');
var fs = require('fs');

var client = amazon.createClient({
    awsId: "XXX",
    awsSecret: "XXX",
    awsTag: "888"
});

var array = fs.readFileSync('./test.txt').toString().split('\n');
for (var i = 1; i < array.length; i++) {
     var ASIN = array[i];

    return client.itemLookup({
            domain: 'webservices.amazon.de',
            responseGroup: 'Large',
            idType: 'ASIN',
            itemId: ASIN
        })
        .then(function(results) {
            fs.writeFile(ASIN + '.json', JSON.stringify(results), function(err) {
                if (err) {
                    console.log(err);
                } else {
                    console.log("JSON saved");
                }
            })

            return results;

        }).catch(function(err) {
            console.log(err);
        });
};

Solution 1:

As of 2019...

...the correct answer is to use async/await with the native fs promises module included in node. Upgrade to Node.js 10 or 11 (already supported by major cloud providers) and do this:

const fs = require('fs').promises;

// This must run inside a function marked `async`:
const file = await fs.readFile('filename.txt', 'utf8');
await fs.writeFile('filename.txt', 'test');

Do not use third-party packages and do not write your own wrappers, that's not necessary anymore.

No longer experimental

Before Node 11.14.0, you would still get a warning that this feature is experimental, but it works just fine and it's the way to go in the future. Since 11.14.0, the feature is no longer experimental and is production-ready.

What if I prefer import instead of require?

It works, too - but only in Node.js versions where this feature is not marked as experimental.

import { promises as fs } from 'fs';

(async () => {
    await fs.writeFile('./test.txt', 'test', 'utf8');
})();

Solution 2:

say

const util = require('util')
const fs_writeFile = util.promisify(fs.writeFile)

https://nodejs.org/api/util.html#util_util_promisify_original

this is less prone to bugs than the top-voted answer

Solution 3:

Because fs.writefile is a traditional asynchronous callback - you need to follow the promise spec and return a new promise wrapping it with a resolve and rejection handler like so:

return new Promise(function(resolve, reject) {
    fs.writeFile("<filename.type>", data, '<file-encoding>', function(err) {
        if (err) reject(err);
        else resolve(data);
    });
});

So in your code you would use it like so right after your call to .then():

 .then(function(results) {
    return new Promise(function(resolve, reject) {
            fs.writeFile(ASIN + '.json', JSON.stringify(results), function(err) {
               if (err) reject(err);
               else resolve(data);
            });
    });
  }).then(function(results) {
       console.log("results here: " + results)
  }).catch(function(err) {
       console.log("error here: " + err);
  });

Solution 4:

Finally, the latest node.js release v10.3.0 has natively supported fs promises.

const fsPromises = require('fs').promises; // or require('fs/promises') in v10.0.0
fsPromises.writeFile(ASIN + '.json', JSON.stringify(results))
  .then(() => {
    console.log('JSON saved');
  })
  .catch(er => {
    console.log(er);
  });

You can check the official documentation for more details. https://nodejs.org/api/fs.html#fs_fs_promises_api