Writing files in Node js

Question

I ve been trying to find a way to write to a file when using Node js  but with no success  How can I do that

User · Answer

Currently there are three ways to write a file:

fs.write(fd, buffer, offset, length, position, callback)

You need to wait for the callback to ensure that the buffer is written to disk. It's not buffered.
fs.writeFile(filename, data, [encoding], callback)

All data must be stored at the same time; you cannot perform sequential writes.
fs.createWriteStream(path, [options])

Creates a WriteStream, which is convenient because you don't need to wait for a callback. But again, it's not buffered.

A WriteStream, as the name says, is a stream. A stream by definition is “a buffer” containing data which moves in one direction (source ? destination). But a writable stream is not necessarily “buffered”. A stream is “buffered” when you write n times, and at time n+1, the stream sends the buffer to the kernel (because it's full and needs to be flushed).

In other words: “A buffer” is the object. Whether or not it “is buffered” is a property of that object.

If you look at the code, the WriteStream inherits from a writable Stream object. If you pay attention, you’ll see how they flush the content; they don't have any buffering system.

If you write a string, it’s converted to a buffer, and then sent to the native layer and written to disk. When writing strings, they're not filling up any buffer. So, if you do:

write("a")
write("b")
write("c")

You're doing:

fs.write(new Buffer("a"))
fs.write(new Buffer("b"))
fs.write(new Buffer("c"))

That’s three calls to the I/O layer. Although you're using “buffers”, the data is not buffered. A buffered stream would do: fs.write(new Buffer ("abc")), one call to the I/O layer.

As of now, in Node.js v0.12 (stable version announced 02/06/2015) now supports two functions: cork() and uncork(). It seems that these functions will finally allow you to buffer/flush the write calls.

For example, in Java there are some classes that provide buffered streams (BufferedOutputStream, BufferedWriter...). If you write three bytes, these bytes will be stored in the buffer (memory) instead of doing an I/O call just for three bytes. When the buffer is full the content is flushed and saved to disk. This improves performance.

I'm not discovering anything, just remembering how a disk access should be done.

User · Answer

I know the question asked about  write  but in a more general sense  append  might be useful in some cases as it is easy to use in a loop to add text to a file  whether the file exists or not   Use a   n  if you want to add lines eg   var fs   require  fs    for  var i 0  i lt 10  i         fs appendFileSync  junk csv    Line   i   n

User · Answer

There are a lot of details in the File System API  The most common way is   const fs   require  fs     fs writeFile   tmp test    Hey there    function err        if err            return console log err             console log  The file was saved              Or fs writeFileSync   tmp test-sync    Hey there

User · Answer

You can write to files with streams  Just do it like this  const fs   require  fs     const stream   fs createWriteStream    test txt    stream write  quot Example text quot

User · Answer

var fs   require  fs     fs writeFile path      message txt    Hello   function err    if  err  throw err    console log  success           For example   read file and write to another file      var fs   require  fs        var path   process cwd        fs readFile path    from txt  function err data                                        if err                          console log err                      else                                                       fs writeFile path    to text  function erro                                   if erro                                      console log  error     erro                                   else                                     console log  success

User · Answer

The answers provided are dated and a newer way to do this is   const fsPromises   require  fs   promises await fsPromises writeFile   path to file txt    data to write     see documents here for more info

User · Answer

Synchronous Write  fs writeFileSync file  data   options    fs   require  fs     fs writeFileSync  quot foo txt quot    quot bar quot     Asynchronous Write  fs writeFile file  data   options   callback   fs   require  fs     fs writeFile  foo txt    bar    err    gt    if  err  throw err       Where file  lt string gt     lt Buffer gt     lt URL gt     lt integer gt  filename or file descriptor data  lt string gt     lt Buffer gt     lt Uint8Array gt  options  lt Object gt     lt string gt  callback  lt Function gt   Worth reading the offical File System  fs  docs  Update  async await fs   require  fs    util   require  util    writeFile   util promisify fs writeFile    fn   async      gt    await writeFile  foo txt    bar       fn

User · Answer

Here we use w  for read write both actions and if the file path is not found the it would be created automatically   fs open path   w    function err  data        if  err            console log  ERROR        err         else           fs write data   content   0   content length   null  function err                if  err                  console log  ERROR        err               fs close data  function                     console log  written success                                           Content means what you have to write to the file and its length   content length

User · Answer

Point 1  If you want to write something into a file  means  it will remove anything already saved in the file and write the new content  use fs promises writeFile   Point 2  If you want to append something into a file  means  it will not remove anything already saved in the file but append the new item in the file content then first read the file  and then add the content into the readable value  then write it to the file  so use fs promises readFile and fs promises writeFile    example 1  I want to write a JSON object in my JSON file   const fs   require  fs     writeFile   filename  writedata  async function writeFile   filename  writedata    try   await fs promises writeFile filename  JSON stringify writedata null  4    utf8    return true   catch err    return false

User · Answer

You may write to a file using fs  file system  module   Here is an example of how you may do it   const fs   require  fs     const writeToFile    fileName  callback    gt      fs open fileName   wx    error  fileDescriptor    gt        if   error  amp  amp  fileDescriptor             Do something with the file here           fs writeFile fileDescriptor  newData   error    gt            if   error              fs close fileDescriptor   error    gt                if   error                  callback false                 else                 callback  Error closing the file                                          else             callback  Error writing to new file                              else         callback  Could not create new file  it may already exists                     You might also want to get rid of this callback-inside-callback code structure by useing Promises and async await statements  This will make asynchronous code structure much more flat  For doing that there is a handy util promisify original  function might be utilized  It allows us to switch from callbacks to promises  Take a look at the example with fs functions below      Dependencies  const util   require  util    const fs   require  fs        Promisify  error-back  functions  const fsOpen   util promisify fs open   const fsWrite   util promisify fs writeFile   const fsClose   util promisify fs close       Now we may create  async  function with  await s  async function doSomethingWithFile fileName      const fileDescriptor   await fsOpen fileName   wx          Do something with the file here       await fsWrite fileDescriptor  newData     await fsClose fileDescriptor

User · Answer

You can write in a file by the following code example   var data       test    123    test2    Lorem Ipsem       fs open datapath     data topplayers json    wx   function  error  fileDescriptor      if   error  amp  amp  fileDescriptor        var stringData   JSON stringify data       fs writeFile fileDescriptor  stringData  function  error          if   error            fs close fileDescriptor  function  error              if   error                callback false               else               callback  Error in close file                                    else           callback  Error in writing file

User · Answer

You can use library easy-file-manager  install first from npm npm install easy-file-manager  Sample to upload and remove files  var filemanager   require  easy-file-manager   var path     public  var filename    test jpg  var data     buffered image  filemanager upload path filename data function err       if  err  console log err        filemanager remove path  aa filename function isSuccess       if  err  console log err

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You can of course make it a little more advanced  Non-blocking  writing bits and pieces  not writing the whole file at once   var fs   require  fs    var stream   fs createWriteStream  my file txt    stream once  open   function fd      stream write  My first row n      stream write  My second row n      stream end

User · Answer

OK  it s quite simple as Node has built-in functionality for this  it s called fs which stands for File System and basically  NodeJS File System module     So first require it in your server js file like this   var fs   require  fs      fs has few methods to do write to file  but my preferred way is using appendFile  this will append the stuff to the file and if the file doesn t exist  will create one  the code could be like below   fs appendFile  myFile txt    Hi Ali    function  err      if  err  throw err    console log  Thanks  It  s saved to the file

User · Answer

I liked Index of   articles file-system   It worked for me   See also How do I write files in node js    fs   require  fs    fs writeFile  helloworld txt    Hello World    function  err        if  err           return console log err       console log  Wrote Hello World in file helloworld txt  just check it          Contents of helloworld txt   Hello World    Update  As in Linux node write in current directory   it seems in some others don t  so I add this comment just in case   Using this ROOT APP PATH   fs realpathSync       console log ROOT APP PATH   to get where the file is written

User · Answer

var path    public uploads file txt   buffer   new Buffer  some content n     fs open path   w   function err  fd        if  err            throw  error opening file      err             fs write fd  buffer  0  buffer length  null  function err            if  err  throw  error writing file      err          fs close fd  function                 console log  file written

User · Answer

Here is the sample of how to read file csv from local and write csv file to local   var csvjson   require  csvjson        fs   require  fs        mongodb   require  mongodb        MongoClient   mongodb MongoClient      mongoDSN    mongodb   localhost 27017 test       collection   function uploadcsvModule        var data   fs readFileSync    home limitless Downloads orders sample csv     encoding    utf8         var importOptions             delimiter           optional          quote              optional        ExportOptions             delimiter                  wrap          false           var myobj   csvjson toSchemaObject data  importOptions      var exportArr       importArr           myobj forEach d  gt           if d orderId  undefined    d orderId                    exportArr push d            else               importArr push d                       var csv   csvjson toCSV exportArr  ExportOptions       MongoClient connect mongoDSN  function error  db            collection   db collection  orders           collection insertMany importArr  function err result               fs writeFile   home limitless Downloads orders sample1 csv   csv    encoding    utf8                 db close                                      uploadcsvModule

User · Answer

Point 1  If you want to write something into a file  means  it will remove anything already saved in the file and write the new content  use fs promises writeFile   Point 2  If you want to append something into a file  means  it will not remove anything already saved in the file but append the new item in the file content then first read the file  and then add the content into the readable value  then write it to the file  so use fs promises readFile and fs promises writeFile    example 1  I want to write a JSON object in my JSON file   const fs   require  fs    writeFile      my data json     id 1  name  my name     async function writeFile   filename  writedata        try           await fs promises writeFile filename  JSON stringify writedata null  4    utf8           console log   data is written successfully in the file             catch err            console log   not able to write data in the file              example2   if you want to append data to a JSON file  you want to add data  id 1  name  my name   to file my data json on the same folder root  just call append data  file path   data   function  It will append data in the JSON file if the file existed   or it will create the file and add the data to it     const fs   require  fs       data    id 1  name  my name      file path      my data json     append data  file path   data       async function append data  filename   data            if  fs existsSync filename              read data   await readFile filename           if  read data    false                 console log  not able to read file                       else                read data push data               dataWrittenStatus   await writeFile filename  read data               if dataWrittenStatus    true                  console log  data added successfully                              else                 console log  data adding failed                                  else            dataWrittenStatus   await writeFile filename   data             if dataWrittenStatus    true                console log  data added successfully                         else               console log  data adding failed                               async function readFile   filePath      try       const data   await fs promises readFile filePath   utf8       return JSON parse data       catch err         return false         async function writeFile   filename  writedata      try         await fs promises writeFile filename  JSON stringify writedata null  4    utf8          return true       catch err          return false

User · Answer

fs createWriteStream path  options   options may also include a start option to allow writing data at some position past the beginning of the file  Modifying a file rather than replacing it may require a flags mode of r  rather than the default mode w  The encoding can be any one of those accepted by Buffer  If autoClose is set to true  default behavior  on  error  or  finish  the file descriptor will be closed automatically  If autoClose is false  then the file descriptor won t be closed  even if there s an error  It is the application s responsibility to close it and make sure there s no file descriptor leak  Like ReadStream  if fd is specified  WriteStream will ignore the path argument and will use the specified file descriptor  This means that no  open  event will be emitted  fd should be blocking  non-blocking fds should be passed to net Socket  If options is a string  then it specifies the encoding   After  reading this long article  You should understand how it works  So  here s an example of createWriteStream       The fs createWriteStream   returns an  WritableStream  aka  internal Writeable  and we want the encoding as  utf -8       The WriteableStream has the method write      fs createWriteStream  out txt    utf-8    write  hello world

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