Using Promises with fs.readFile in a loop

Question

I'm trying to understand why the below promise setups don't work.

(Note: I already solved this issue with async.map. But I would like to learn why my attempts below didn't work.)

The correct behavior should be: bFunc should run as many time as necessary to fs read all the image files (bFunc below runs twice) and then cFunc console prints "End".

Thanks!

Attempt 1: It runs and stops at cFunc().

var fs = require('fs');

bFunc(0)
.then(function(){ cFunc() }) //cFunc() doesn't run

function bFunc(i){
    return new Promise(function(resolve,reject){

        var imgPath = __dirname + "/image1" + i + ".png";

        fs.readFile(imgPath, function(err, imagebuffer){

            if (err) throw err;
            console.log(i)

            if (i<1) {
                i++;
                return bFunc(i);
            } else {
                resolve();
            };

        });

    })
}

function cFunc(){
    console.log("End");
}

Attempt 2: In this case, I used a for-loop but it executes out of order. Console prints: End, bFunc done, bFunc done

var fs = require('fs');

bFunc()
        .then(function(){ cFunc() })

function bFunc(){
    return new Promise(function(resolve,reject){

        function read(filepath) {
            fs.readFile(filepath, function(err, imagebuffer){
                if (err) throw err;
                console.log("bFunc done")
            });
        }

        for (var i=0; i<2; i++){
            var imgPath = __dirname + "/image1" + i + ".png";
            read(imgPath);
        };

        resolve()
    });
}


function cFunc(){
    console.log("End");
}

Thanks for the help in advance!

Answer 1

So, anytime you have multiple async operations to coordinate in some way, I immediately want to go to promises. And, the best way to use promises to coordinate a number of async operations is to make each async operation return a promise. The lowest level async operation you show is fs.readFile(). Since I use the Bluebird promise library, it has a function for "promisifying" a whole module's worth of async functions.

var Promise = require('bluebird');
var fs = Promise.promisifyAll(require('fs'));

This will create new parallel methods on the fs object with an "Async" suffix that return promises instead of use straight callbacks. So, there will be an fs.readFileAsync() that returns a promise. You can read more about Bluebird's promisification here.

So, now you can make a function that gets an image fairly simply and returns a promise whose value is the data from the image:

 function getImage(index) {
     var imgPath = __dirname + "/image1" + index + ".png";
     return fs.readFileAsync(imgPath);
 }

Then, in your code, it looks like you want to make bFunc() be a function that reads three of these images and calls cFunc() when they are done. You can do that like this:

var Promise = require('bluebird');
var fs = Promise.promisifyAll(require('fs'));

 function getImage(index) {
     var imgPath = __dirname + "/image1" + index + ".png";
     return fs.readFileAsync(imgPath);
 }

 function getAllImages() {
    var promises = [];
    // load all images in parallel
    for (var i = 0; i <= 2; i++) {
        promises.push(getImage(i));
    }
    // return promise that is resolved when all images are done loading
    return Promise.all(promises);
 }

 getAllImages().then(function(imageArray) {
    // you have an array of image data in imageArray
 }, function(err) {
    // an error occurred
 });

If you did not want to use Bluebird, you could manually make a promise version of fs.readFile() like this:

// make promise version of fs.readFile()
fs.readFileAsync = function(filename) {
    return new Promise(function(resolve, reject) {
        fs.readFile(filename, function(err, data){
            if (err) 
                reject(err); 
            else 
                resolve(data);
        });
    });
};

Or, in modern versions of node.js, you can use util.promisify() to make a promisified version of a function that follows the node.js async calling convention:

const util = require('util');
fs.readFileAsync = util.promisify(fs.readFile);

Though, you will quickly find that once you start using promises, you want to use them for all async operations so you'll be "promisifying" lots of things and having a library or at least a generic function that will do that for you will save lots of time.

---

In even newer versions of node.js (version 10.0+), you can use the built-in version of the fs library that supports promises:

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

fsp.readFile("someFile").then(data => {
    console.log(data);
});

Answer 2

Node v10 has fs Promises API

const fsPromises = require('fs').promises

const func = async filenames => {

  for(let fn of filenames) {
    let data = await fsPromises.readFile(fn)
  }

}

func(['file1','file2'])
  .then(res => console.log('all read', res))
  .catch(console.log)

https://nodejs.org/api/fs.html#fs_fs_promises_api

Or if you want to read more files simultaneously:

const func = filenames => {
  return Promise.all(
    filenames.map(f => fsPromises.readFile(f))
  )
}

func(['./a','./b'])
  .then(res => console.log('all read', res))
  .catch(console.log)

Answer 3

Your code should look more like this:

// promisify fs.readFile()
fs.readFileAsync = function (filename) {
    return new Promise((resolve, reject) => {
        fs.readFile(filename, (err, buffer) => {
            if (err) reject(err); else resolve(buffer);
        });
    });
};

const IMG_PATH = "foo";

// utility function
function getImageByIdAsync(i) {
    return fs.readFileAsync(IMG_PATH + "/image1" + i + ".png");
}

Usage with a single image:

getImageByIdAsync(0).then(imgBuffer => {
    console.log(imgBuffer);
}).catch(err => {
    console.error(err);
});

Usage with multiple images:

var images = [1,2,3,4].map(getImageByIdAsync);

Promise.all(images).then(imgBuffers => {
    // all images have loaded
}).catch(err => {
    console.error(err);
});

To promisify a function means to take an asynchronous function with callback semantics and derive from it a new function with promise semantics.

It can be done manually, like shown above, or – preferably – automatically. Among others, the Bluebird promise library has a helper for that, see http://bluebirdjs.com/docs/api/promisification.html

Answer 4

If you're using .mjs ECMAScript Modules import syntax then here's code that reads a file, based on this GitHub gist comment reply:

import { promises as fs } from 'fs';
let json = await fs.readFile('./package.json', 'utf-8');
console.log(json);

Here's code that does multiple files, based on this answer:

import { promises as fs } from 'fs';

const sequentially = async filenames => {
    for(let fn of filenames) {
        let json = await fs.readFile(fn, 'utf-8');
        console.log(json);
    }
}

const parallel = filenames => {
    return Promise.all(
      filenames.map(fn => fs.readFile(fn, 'utf-8'))
    )
  }

const fns = ['package.json', 'package-lock.json'];
await sequentially(fns);
await parallel(fns);
console.log('all read');

Answer 5

you can also use this module: 'fs-readfile-promise'

var readFile = require('fs-readfile-promise');
readFile(__dirname + '/file1.txt','utf-8').then(function (data){
    console.log("file's name:", data)
    return readFile(__dirname +'/'+data, 'utf-8')
}).then(function (data1){
    console.log('Content data:', data1)
}).catch( function (err){
    console.log(err)
})

Answer 6

I once wrote a library for using promises in a loop. It's called for-async. It's pretty small but getting all the details right is hard, so you can have a look at this for inspiration maybe.

This mainly takes care of running the promise-returning functions in sequence, waiting for the promise to resolve before proceeding to the next item and of handling errors. This differs from Promise.map because that function runs the promise-returning functions in parallel, which may or may not be what you want.

// example: looping over promise-returning functions with forAsync
forAsync(
  ['a.txt', 'b.txt', 'c.txt'], 
  readFile
)

// worker function that returns a promise
function readFile(name, idx) {
  return new Promise(function(resolve, reject) {
    setTimeout(function(){
      console.info('read file ' + idx + ': ' + name)
      resolve()
    }, 1000)
  })
}

// forAsync
function forAsync(arr, work) {
  function loop(arr, i) {
    return new Promise(function (resolve, reject) {
      if (i >= arr.length) {
        resolve();
      } else try {
        Promise.resolve(work(arr[i], i)).then(function () {
          return resolve(loop(arr, i + 1));
        }).catch(reject);
      } catch (error) {
        reject(error);
      }
    });
  }
  return loop(arr, 0);
}