Using filesystem in node.js with async / await

I would like to use async/await with some filesystem operations. Normally async/await works fine because I use babel-plugin-syntax-async-functions.

But with this code I run into the if case where names is undefined:

import fs from 'fs';

async function myF() {
  let names;
  try {
    names = await fs.readdir('path/to/dir');
  } catch (e) {
    console.log('e', e);
  }
  if (names === undefined) {
    console.log('undefined');
  } else {
    console.log('First Name', names[0]);
  }
}

myF();

When I rebuild the code into the callback hell version everything is OK and I get the filenames. Thanks for your hints.

Solution 1:

Native support for async await fs functions since Node 11

Since Node.JS 11.0.0 (stable), and version 10.0.0 (experimental), you have access to file system methods that are already promisify'd and you can use them with try catch exception handling rather than checking if the callback's returned value contains an error.

The API is very clean and elegant! Simply use .promises member of fs object:

import fs from 'fs';
const fsPromises = fs.promises;

async function listDir() {
  try {
    return fsPromises.readdir('path/to/dir');
  } catch (err) {
    console.error('Error occured while reading directory!', err);
  }
}

listDir();

Solution 2:

Starting with node 8.0.0, you can use this:

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

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

async function myF() {
  let names;
  try {
    names = await readdir('path/to/dir');
  } catch (err) {
    console.log(err);
  }
  if (names === undefined) {
    console.log('undefined');
  } else {
    console.log('First Name', names[0]);
  }
}

myF();

See https://nodejs.org/dist/latest-v8.x/docs/api/util.html#util_util_promisify_original

Solution 3:

Node.js 8.0.0

Native async / await

Promisify

From this version, you can use native Node.js function from util library.

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

const readFileAsync = promisify(fs.readFile)
const writeFileAsync = promisify(fs.writeFile)

const run = async () => {
  const res = await readFileAsync('./data.json')
  console.log(res)
}

run()

Promise Wrapping

const fs = require('fs')

const readFile = (path, opts = 'utf8') =>
  new Promise((resolve, reject) => {
    fs.readFile(path, opts, (err, data) => {
      if (err) reject(err)
      else resolve(data)
    })
  })

const writeFile = (path, data, opts = 'utf8') =>
  new Promise((resolve, reject) => {
    fs.writeFile(path, data, opts, (err) => {
      if (err) reject(err)
      else resolve()
    })
  })

module.exports = {
  readFile,
  writeFile
}

...


// in some file, with imported functions above
// in async block
const run = async () => {
  const res = await readFile('./data.json')
  console.log(res)
}

run()

Advice

Always use try..catch for await blocks, if you don't want to rethrow exception upper.

Solution 4:

As of v10.0, you can use fs.Promises

Example using readdir

const { promises: fs } = require("fs");

async function myF() {
    let names;
    try {
        names = await fs.readdir("path/to/dir");
    } catch (e) {
        console.log("e", e);
    }
    if (names === undefined) {
        console.log("undefined");
    } else {
        console.log("First Name", names[0]);
    }
}

myF();

Example using readFile

const { promises: fs } = require("fs");

async function getContent(filePath, encoding = "utf-8") {
    if (!filePath) {
        throw new Error("filePath required");
    }

    return fs.readFile(filePath, { encoding });
}

(async () => {
    const content = await getContent("./package.json");

    console.log(content);
})();

Solution 5:

You might produce the wrong behavior because the File-Api fs.readdir does not return a promise. It only takes a callback. If you want to go with the async-await syntax you could 'promisify' the function like this:

function readdirAsync(path) {
  return new Promise(function (resolve, reject) {
    fs.readdir(path, function (error, result) {
      if (error) {
        reject(error);
      } else {
        resolve(result);
      }
    });
  });
}

and call it instead:

names = await readdirAsync('path/to/dir');