How to read file with async await properly

Question

I cannot figure out how async await works  I slightly understands it but I can t make it work   function loadMonoCounter         fs readFileSync  monolitic txt    binary   async function err  data           return await new Buffer  data            module exports read   function       console log loadMonoCounter          I know I could use readFileSync  but if I do  I know I ll never understand async await and I ll just bury the issue   Goal  Call loadMonoCounter   and return the content of a file   That file is incremented every time incrementMonoCounter   is called  every page load   The file contain the dump of a buffer in binary and is stored on a SSD   No matter what I do  I get an error or undefined in the console

User · Answer

To keep it succint and retain all functionality of fs:

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

async function loadMonoCounter() {
    const data = await fsPromises.readFile('monolitic.txt', 'binary');
    return new Buffer(data);
}

Importing fs and fs.promises separately will give access to the entire fs API while also keeping it more readable... So that something like the next example is easily accomplished.

// the 'next example'
fsPromises.access('monolitic.txt', fs.constants.R_OK | fs.constants.W_OK)
    .then(() => console.log('can access'))
    .catch(() => console.error('cannot access'));

User · Answer

You can use fs promises available natively since Node v11 0 0  import fs from  fs    const readFile   async filePath   gt      try       const data   await fs promises readFile filePath   utf8       return data       catch err        console log err

User · Answer

To use await async you need methods that return promises  The core API functions don t do that without wrappers like promisify   const fs   require  fs    const util   require  util        Convert fs readFile into Promise version of same     const readFile   util promisify fs readFile    function getStuff       return readFile  test          Can t use  await  outside of an async function so you need to chain    with then   getStuff   then data   gt      console log data        As a note  readFileSync does not take a callback  it returns the data or throws an exception  You re not getting the value you want because that function you supply is ignored and you re not capturing the actual return value

User · Answer

You can easily wrap the readFile command with a promise like so   async function readFile path        return new Promise  resolve  reject    gt          fs readFile path   utf8   function  err  data            if  err              reject err                     resolve data                           then use   await readFile  path to file

User · Answer

There is a fs readFileSync  path  options   method  which is synchronous

User · Answer

Since Node v11 0 0 fs promises are available natively without promisify   const fs   require  fs   promises  async function loadMonoCounter         const data   await fs readFile  monolitic txt    binary        return new Buffer data

User · Answer

This is TypeScript version of  Joel s answer  It is usable after Node 11 0   import   promises as fs   from  fs    async function loadMonoCounter         const data   await fs readFile  monolitic txt    binary        return Buffer from data

[node.js] How to read file with async/await properly?

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