Get data from fs readFile

Question

var content  fs readFile    Index html   function read err  data        if  err            throw err            content   data      console log content     Logs undefined  why

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var path    quot index html quot   const readFileAsync   fs readFileSync path   utf8       console log readFileAsync   using simple readFileSync works for me

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var content  fs readFile    Index html   function read err  data        if  err            throw err            content   data      console log content     This is just because node is asynchronous and it will not wait for the read function and as soon as the program starts it will console the value as undefined  Which is actually true because there is no value assigned to content variable  To handle we can use promises  generators etc   We can use promise in this way   new Promise  resolve reject   gt       fs readFile    index html   utf-8   err  data   gt           if  err                reject err      in the case of error  control flow goes to the catch block with the error occured                    else              resolve data       in the case of success  control flow goes to the then block with the content of the file                        then  data   gt       console log data      use your content of the file here  in this then           catch  err   gt       throw err      handle error here

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var fs   require  fs    var path    process cwd      text txt     fs readFile path   function err data        if err          console log err      else         console log data toString

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const fs   require  fs   function readDemo1 file1        return new Promise function  resolve  reject            fs readFile file1   utf8   function  err  dataDemo1                if  err                  reject err               else                 resolve dataDemo1                         async function copyFile          try           let dataDemo1   await readDemo1  url           dataDemo1      n     await readDemo1  url            await writeDemo2 dataDemo1          console log dataDemo1        catch  error            console error error           copyFile     function writeDemo2 dataDemo1        return new Promise function resolve  reject          fs writeFile  text txt   dataDemo1   utf8   function err            if  err            reject err           else           resolve  Promise Success

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There is actually a Synchronous function for this  http   nodejs org api fs html fs fs readfilesync filename encoding Asynchronous fs readFile filename   encoding    callback   Asynchronously reads the entire contents of a file   Example  fs readFile   etc passwd   function  err  data      if  err  throw err    console log data        The callback is passed two arguments  err  data   where data is the contents of the file  If no encoding is specified  then the raw buffer is returned   SYNCHRONOUS fs readFileSync filename   encoding   Synchronous version of fs readFile  Returns the contents of the file named filename  If encoding is specified then this function returns a string  Otherwise it returns a buffer  var text   fs readFileSync  test md   utf8   console log  text

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sync and async file reading way     fs module to read file in sync and async way  var fs   require  fs        filePath      sample files sample css css       this for async way   fs readFile filePath   utf8   function  err  data        if  err  throw err      console log data            this is sync way var css   fs readFileSync filePath   utf8    console log css     Node Cheat Available at read file

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function readContent callback        fs readFile    Index html   function  err  content            if  err  return callback err          callback null  content            readContent function  err  content        console log content

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The following is function would work for async wrap or promise then chains  const readFileAsync    async  path    gt  fs readFileSync path   utf8

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Use the built in promisify library  Node 8   to make these old callback functions more elegant   const fs   require  fs    const util   require  util     const readFile   util promisify fs readFile    async function doStuff       try       const content   await readFile filePath   utf8        console log content       catch  e        console error e

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Using Promises with ES7  Asynchronous use with mz fs  The mz module provides promisified versions of the core node library  Using them is simple  First install the library     npm install mz   Then     const fs   require  mz fs    fs readFile    Index html   then contents   gt  console log contents      catch err   gt  console error err      Alternatively you can write them in asynchronous functions   async function myReadfile        try       const file   await fs readFile    Index html          catch  err    console error  err

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To put it roughly  you re dealing with node js which is asynchronous in nature   When we talk about async  we re talking about doing or processing info or data while dealing with something else  It is not synonymous to parallel  please be reminded   Your code    var content  fs readFile    Index html   function read err  data        if  err            throw err            content   data      console log content     With your sample  it basically does the console log part first  thus the variable  content  being undefined   If you really want the output  do something like this instead   var content  fs readFile    Index html   function read err  data        if  err            throw err            content   data      console log content         This is asynchronous  It will be hard to get used to but  it is what it is  Again  this is a rough but fast explanation of what async is

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As said  fs readFile is an asynchronous action  It means that when you tell node to read a file  you need to consider that it will take some time  and in the meantime  node continued to run the following code  In your case it s  console log content     It s like sending some part of your code for a long trip  like reading a big file    Take a look at the comments that I ve written   var content      node  go fetch this file  when you come back  please run this  read  callback function fs readFile    Index html   function read err  data        if  err            throw err            content   data          in the meantime  please continue and run this console log console log content     That s why content is still empty when you log it  node has not yet retrieved the file s content   This could be resolved by moving console log content  inside the callback function  right after content   data   This way you will see the log when node is done reading the file and after content gets a value

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var data   fs readFileSync  tmp reltioconfig json   utf8      use this for calling a file synchronously  without encoding its showing output as a buffer

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This line will work   const content   fs readFileSync    Index html    utf8    console log content

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you can read file by  var readMyFile   function path  cb          fs readFile path   utf8   function err  content            if  err  return cb err  null           cb null  content                      Adding on you can write to file   var createMyFile    path  data  cb    gt      fs writeFile path  data  function err        if  err  return console error err       cb               and even chain it together  var readFileAndConvertToSentence   function path  callback      readMyFile path  function err  content        if  err          callback err  null         else         var sentence   content split   n   join             callback null  sentence

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To elaborate on what  Raynos said  the function you have defined is an asynchronous callback  It doesn t execute right away  rather it executes when the file loading has completed  When you call readFile  control is returned immediately and the next line of code is executed  So when you call console log  your callback has not yet been invoked  and this content has not yet been set  Welcome to asynchronous programming   Example approaches   const fs   require  fs       First I want to read the file fs readFile    Index html   function read err  data        if  err            throw err            const content   data          Invoke the next step here however you like     console log content        Put all of the code here  not the best solution      processFile content        Or put the next step in a function and invoke it      function processFile content        console log content       Or better yet  as Raynos example shows  wrap your call in a function and pass in your own callbacks   Apparently this is better practice  I think getting into the habit of wrapping your async calls in function that takes a callback will save you a lot of trouble and messy code   function doSomething  callback           any async callback invokes callback with response    doSomething  function doSomethingAfter err  result           process the async result

[javascript] Get data from fs.readFile

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