Using filesystem in node js with async await

Question

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

User · Answer

This is the TypeScript version to the question  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

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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

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I have this little helping module that exports promisified versions of fs functions  const fs   require  fs    const  promisify    require  util    module exports       readdir  promisify fs readdir     readFile  promisify fs readFile     writeFile  promisify fs writeFile       etc

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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

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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      gt      const res   await readFileAsync    data json     console log res     run        Promise Wrapping  const fs   require  fs    const readFile    path  opts    utf8     gt    new Promise  resolve  reject    gt        fs readFile path  opts   err  data    gt          if  err  reject err        else resolve data               const writeFile    path  data  opts    utf8     gt    new Promise  resolve  reject    gt        fs writeFile path  data  opts   err    gt          if  err  reject err        else resolve                module exports       readFile    writeFile             in some file  with imported functions above    in async block const run   async      gt      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

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You can use the simple and lightweight module https   github com nacholibre nwc-l it supports both async and sync methods  Note  this module was created by me

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Node v14 0 0 and above you can just do  import   readdir   from  quot fs promises quot    just like you would import from  quot fs quot  see this PR for more details  https   github com nodejs node pull 31553

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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

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Recommend using an npm package such as https   github com davetemplin async-file  as compared to custom functions   For example    import   as fs from  async-file    await fs rename   tmp hello     tmp world    await fs appendFile  message txt    data to append    await fs access   etc passd   fs constants R OK   fs constants W OK    var stats   await fs stat   tmp hello     tmp world      Other answers are outdated

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As of v10 0  you can use fs Promises Example using readdir const   promises  fs     require  quot fs quot     async function myF         let names      try           names   await fs readdir  quot path to dir quot          catch  e            console log  quot e quot   e             if  names     undefined            console log  quot undefined quot          else           console log  quot First Name quot   names 0             myF     Example using readFile const   promises  fs     require  quot fs quot     async function getContent filePath  encoding    quot utf-8 quot         if   filePath            throw new Error  quot filePath required quot               return fs readFile filePath    encoding         async      gt        const content   await getContent  quot   package json quot         console log content

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Here is what worked for me   const fsp   require  fs-promise      async      gt      try       const names   await fsp readdir  path to dir        console log names 0        catch  e        console log  error     e               This code works in node 7 6 without babel when harmony flag is enabled  node --harmony my-script js  And starting with node 7 7  you don t even need this flag   The fsp library included in the beginning is just a promisified wrapper for fs  and fs-ext    I   m really exited about what you can do in node without babel these days  Native async await make writing code such a pleasure   UPDATE 2017-06  fs-promise module was deprecated  Use fs-extra instead with the same API

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