How do I read the contents of a Node js stream into a string variable

Question

I m hacking on a Node program that uses smtp-protocol to capture SMTP emails and act on the mail data  The library provides the mail data as a stream  and I don t know how to get that into a string   I m currently writing it to stdout with stream pipe process stdout    end  false     but as I said  I need the stream data in a string instead  which I can use once the stream has ended   How do I collect all the data from a Node js stream into a string

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I had more luck using like that    let string       readstream      on  data    buf    gt  string    buf toString         on  end        gt  console log string      I use node v9 11 1 and the readstream is the response from a http get callback

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The cleanest solution may be to use the  string-stream  package  which converts a stream to a string with a promise   const streamString   require  stream-string    streamString myStream  then string variable   gt           myStream was converted to a string  and that string is stored in string variable     console log string variable      catch err   gt            myStream emitted an error event  err   so the promise from stream-string was rejected     throw err

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From the nodejs documentation you should do this - always remember a string without knowing the encoding is just a bunch of bytes   var readable   getReadableStreamSomehow    readable setEncoding  utf8    readable on  data   function chunk      assert equal typeof chunk   string      console log  got  d characters of string data   chunk length

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I m using usually this simple function to transform a stream into a string   function streamToString stream  cb      const chunks         stream on  data    chunk    gt        chunks push chunk toString             stream on  end        gt        cb chunks join                 Usage example   let stream   fs createReadStream    myFile foo    streamToString stream   data    gt      console log data       data is now my string variable

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What do you think about this      lets have a ReadableStream as a stream variable  const chunks        for await  let chunk of stream        chunks push chunk     const buffer    Buffer concat chunks   const str   buffer toString  quot utf-8 quot

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setEncoding  utf8     Well done Sebastian J above   I had the  buffer problem  with a few lines of test code I had  and added the encoding information and it solved it  see below   Demonstrate the problem  software     process stdin setEncoding  utf8    process stdin on  data    data    gt        console log typeof data   data         input  hello world   output  object  lt Buffer 68 65 6c 6c 6f 20 77 6f 72 6c 64 0d 0a gt    Demonstrate the solution  software  process stdin setEncoding  utf8        lt - Activate  process stdin on  data    data    gt        console log typeof data   data         input  hello world   output  string hello world

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In my case  the content type response headers was Content-Type  text plain  So  I ve read the data from Buffer like   let data       stream on  data    chunk    gt     console log Buffer from chunk  toString     data push Buffer from chunk  toString

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Another way would be to convert the stream to a promise  refer to the example below  and use then  or await  to assign the resolved value to a variable  function streamToString  stream      const chunks         return new Promise  resolve  reject    gt        stream on  data    chunk    gt  chunks push chunk        stream on  error    err    gt  reject err        stream on  end        gt  resolve Buffer concat chunks  toString  utf8              const result   await streamToString stream

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This answer is from years ago  when it was the best answer  There is now a better answer below this  I haven t kept up with node js  and I cannot delete this answer because it is marked  quot correct on this question quot   If you are thinking of down clicking  what do you want me to do   The key is to use the data and end events of a Readable Stream  Listen to these events  stream on  data    chunk    gt            stream on  end        gt             When you receive the data event  add the new chunk of data to a Buffer created to collect the data  When you receive the end event  convert the completed Buffer into a string  if necessary  Then do what you need to do with it

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Streams don t have a simple  toString   function  which I understand  nor something like a  toStringAsync cb  function  which I don t understand    So I created my own helper function   var streamToString   function stream  callback      var str         stream on  data   function chunk        str    chunk          stream on  end   function         callback str               how to use  streamToString myStream  function myStr      console log myStr

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All the answers listed appear to open the Readable Stream in flowing mode which is not the default in NodeJS and can have limitations since it lacks backpressure support that NodeJS provides in Paused Readable Stream Mode   Here is an implementation using Just Buffers  Native Stream and Native Stream Transforms and support for Object Mode  import  Transform  from  stream    let buffer  null       function objectifyStream         return new Transform           objectMode  true          transform  function chunk  encoding  next                 if   buffer                    buffer   Buffer from     chunk                  else                   buffer   Buffer from     buffer     chunk                              next null  buffer                        process stdin pipe objectifyStream    process stdout

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Using the quite popular stream-buffers package which you probably already have in your project dependencies  this is pretty straightforward      imports const   WritableStreamBuffer     require  stream-buffers    const   promisify     require  util    const   createReadStream     require  fs    const pipeline   promisify require  stream   pipeline       sample stream let stream   createReadStream   etc hosts        pipeline the stream into a buffer  and print the contents when done let buf   new WritableStreamBuffer    pipeline stream  buf  then      gt  console log buf getContents   toString

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Hope this is more useful than the above answer   var string       stream on  data  function data     string    data toString      console log  stream data     part        stream on  end  function      console log  final output     string         Note that string concatenation is not the most efficient way to collect the string parts  but it is used for simplicity  and perhaps your code does not care about efficiency    Also  this code may produce unpredictable failures for non-ASCII text  it assumes that every character fits in a byte   but perhaps you do not care about that  either

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What about something like a stream reducer    Here is an example using ES6 classes how to use one   var stream   require  stream    class StreamReducer extends stream Writable     constructor chunkReducer  initialvalue  cb        super        this reducer   chunkReducer      this accumulator   initialvalue      this cb   cb         write chunk  enc  next        this accumulator   this reducer this accumulator  chunk       next          end         this cb null  this accumulator            just a test stream class EmitterStream extends stream Readable     constructor chunks        super        this chunks   chunks         read         this chunks forEach function  chunk             this push chunk         bind this        this push null             just transform the strings into buffer as we would get from fs stream or http request stream  new EmitterStream      hello     world        map function str         return Buffer from str   utf8            pipe new StreamReducer    function  acc  v        acc push v       return acc               function err  chunks        console log Buffer concat chunks  toString  utf8

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Easy way with the popular  over 5m weekly downloads  and lightweight get-stream library   https   www npmjs com package get-stream  const fs   require  fs    const getStream   require  get-stream      async      gt        const stream   fs createReadStream  unicorn txt        console log await getStream stream      output is string

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This worked for me and is based on Node v6 7 0 docs   let output       stream on  readable   function         let read   stream read        if  read     null               New stream data is available         output    read toString          else              Stream is now finished when read is null             You can callback here e g           callback null  output              stream on  error   function err      callback err  null

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And yet another one for strings using promises  function getStream stream      return new Promise resolve   gt        const chunks              Buffer from is required if chunk is a String  see comments     stream on  quot data quot   chunk   gt  chunks push Buffer from chunk         stream on  quot end quot        gt  resolve Buffer concat chunks  toString                Usage  const stream   fs createReadStream   filename   getStream stream  then r  gt console log r     remove the  toString   to use with binary Data if required  update   AndreiLED correctly pointed out this has problems with strings  I couldn t get a stream returning strings with the version of node I have  but the api notes this is possible

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None of the above worked for me  I needed to use the Buffer object     const chunks          readStream on  data   function  chunk        chunks push chunk               Send the buffer or you can put it into a var   readStream on  end   function          res send Buffer concat chunks

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