node js read a text file into an array Each line an item in the array

Question

I would like to read a very  very large file into a JavaScript array in node js   So  if the file is like this   first line two  three           I would have the array      first line   two   three                  The function would look like this   var array   load filename      Therefore the idea of loading it all as a string and then splitting it is not acceptable

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i just want to add  finbarr great answer  a little fix in the asynchronous example   Asynchronous   var fs   require  fs    fs readFile  file txt   function err  data        if err  throw err      var array   data toString   split   n        for i in array            console log array i              done           MadPhysicist  done   is what releases the async  call

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With a BufferedReader  but the function should be asynchronous   var load   function  file  cb       var lines           new BufferedReader  file    encoding   utf8              on   error   function  error               cb  error  null                       on   line   function  line               lines push  line                       on   end   function                 cb  null  lines                       read         load   file   function  error  lines       if  error  return console log  error       console log  lines

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Synchronous   var fs   require  fs    var array   fs readFileSync  file txt   toString   split   n    for i in array        console log array i        Asynchronous   var fs   require  fs    fs readFile  file txt   function err  data        if err  throw err      var array   data toString   split   n        for i in array            console log array i

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Using the Node js readline module   var fs   require  fs    var readline   require  readline     var filename   process argv 2   readline createInterface       input  fs createReadStream filename       terminal  false    on  line   function line       console log  Line      line

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This is a variation on the answer above by  mtomis   It creates a stream of lines  It emits  data  and  end  events  allowing you to handle the end of the stream   var events   require  events     var LineStream   function  input        var remaining            input on  data   function  data            remaining    data          var index   remaining indexOf   n            var last   0          while  index  gt  -1                var line   remaining substring last  index               last   index   1              this emit  data   line               index   remaining indexOf   n   last                     remaining   remaining substring last         bind this         input on  end   function             if  remaining length  gt  0                this emit  data   remaining                     this emit  end          bind this       LineStream prototype   new events EventEmitter    Use it as a wrapper   var lineInput   new LineStream input    lineInput on  data   function  line           handle line      lineInput on  end   function            wrap it up

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Essentially this will do the job   replace   r n g   n   split   n    This works on Mac  Linux  amp  Windows  Code Snippets Synchronous  const   readFileSync     require  fs     const array   readFileSync  file txt   toString   replace   r n g   n   split   n     for let i of array        console log i      Asynchronous  With the fs promises API that provides an alternative set of asynchronous file system methods that return Promise objects rather than using callbacks   No need to promisify  you can use async-await with this too  available on and after Node js version 10 0 0  const   readFile     require  fs   promises   readFile  file txt   function err  data        if err  throw err       const arr   data toString   replace   r n g   n   split   n         for let i of arr            console log i              More about  r  amp   n here   r n   r and  n what is the difference between them

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Using Node js v8 or later has a new feature that converts normal function into an async function   util promisify  It s an awesome feature  Here s the example of parsing 10000 numbers from the txt file into an array  counting inversions using merge sort on the numbers      read from txt file const util   require  util    const fs   require  fs   fs readFileAsync   util promisify fs readFile   let result       const parseTxt   async  csvFile    gt      let fields  obj   const data   await fs readFileAsync csvFile    const str   data toString     const lines   str split   r n        const lines   str   console log  lines   lines       console log  str   str     lines map line   gt        if  line   return null      result push Number line          console log  result  result    return result   parseTxt    count-inversion txt   then      gt      console log mergeSort  arr  result  count  0

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js   var array   fs readFileSync  file txt    utf8   split   n      ts   var array   fs readFileSync  file txt    utf8   toString   split   n

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use readline  documentation   here s an example reading a css file  parsing for icons and writing them to json  var results         var rl   require  readline   createInterface       input  require  fs   createReadStream    assets stylesheets  icons scss                for every new line  if it matches the regex  add it to an array      this is ugly regex      rl on  line   function  line        var re      icon-icon          var match      if   match   re exec line       null          results push match 0  replace         replace                              readline emits a close event when the file is read    rl on  close   function        var outputFilename      icons json       fs writeFile outputFilename  JSON stringify results  null  2   function err            if err              console log err             else             console log  JSON saved to     outputFilename

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I had the same problem  and I have solved it with the module line-by-line  https   www npmjs com package line-by-line  At least for me works like a charm  both in synchronous and asynchronous mode   Also  the problem with lines terminating not terminating  n can be solved with the option      encoding   utf8   skipEmptyLines  false     Synchronous processing of lines   var LineByLineReader   require  line-by-line        lr   new LineByLineReader  big file txt     lr on  error   function  err            err  contains error object      lr on  line   function  line            line  contains the current line without the trailing newline character       lr on  end   function             All lines are read  file is closed now

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To read a big file into array you can read line by line or chunk by chunk   line by line refer to my answer here  var fs   require  fs        es   require  event-stream     var lines        var s   fs createReadStream  filepath        pipe es split         pipe es mapSync function line              pause the readstream         s pause            lines push line           s resume                on  error   function err            console log  Error    err               on  end   function             console log  Finish reading             console log lines               chunk by chunk refer to this article  var offset   0  var chunkSize   2048  var chunkBuffer   new Buffer chunkSize   var fp   fs openSync  filepath    r    var bytesRead   0  while bytesRead   fs readSync fp  chunkBuffer  0  chunkSize  offset         offset    bytesRead      var str   chunkBuffer slice 0  bytesRead  toString        var arr   str split   n         if bytesRead   chunkSize               the last item of the arr may be not a full line  leave it to the next chunk         offset -  arr pop   length            lines push arr     console log lines

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If you can fit the final data into an array then wouldn t you also be able to fit it in a string and split it  as has been suggested  In any case if you would like to process the file one line at a time you can also try something like this   var fs   require  fs     function readLines input  func      var remaining          input on  data   function data        remaining    data      var index   remaining indexOf   n        while  index  gt  -1          var line   remaining substring 0  index         remaining   remaining substring index   1         func line         index   remaining indexOf   n                   input on  end   function         if  remaining length  gt  0          func remaining                  function func data      console log  Line      data      var input   fs createReadStream  lines txt    readLines input  func     EDIT   in response to comment by phopkins  I think  at least in newer versions  substring does not copy data but creates a special SlicedString object  from a quick glance at the v8 source code   In any case here is a modification that avoids the mentioned substring  tested on a file several megabytes worth of  All work and no play makes Jack a dull boy     function readLines input  func      var remaining          input on  data   function data        remaining    data      var index   remaining indexOf   n        var last    0      while  index  gt  -1          var line   remaining substring last  index         last   index   1        func line         index   remaining indexOf   n   last              remaining   remaining substring last            input on  end   function         if  remaining length  gt  0          func remaining

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file lines with JFile package Pseudo var JFile require  jfile     var myF new JFile  quot   data txt quot    myF lines      quot first line quot   quot second line quot         Don t forget before   npm install jfile --save

[javascript] node.js: read a text file into an array. (Each line an item in the array.)

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