writable stream that concatenates strings or data and calls a callback with the result
Latest commit 731fedd Sep 1, 2016 @mafintosh mafintosh 1.5.2



Writable stream that concatenates all the data from a stream and calls a callback with the result. Use this when you want to collect all the data from a stream into a single buffer.

Build Status



Streams emit many buffers. If you want to collect all of the buffers, and when the stream ends concatenate all of the buffers together and receive a single buffer then this is the module for you.

Only use this if you know you can fit all of the output of your stream into a single Buffer (e.g. in RAM).

There are also objectMode streams that emit things other than Buffers, and you can concatenate these too. See below for details.


stream-each is part of the mississippi stream utility collection which includes more useful stream modules similar to this one.



var fs = require('fs')
var concat = require('concat-stream')

var readStream = fs.createReadStream('cat.png')
var concatStream = concat(gotPicture)

readStream.on('error', handleError)

function gotPicture(imageBuffer) {
  // imageBuffer is all of `cat.png` as a node.js Buffer

function handleError(err) {
  // handle your error appropriately here, e.g.:
  console.error(err) // print the error to STDERR
  process.exit(1) // exit program with non-zero exit code


var write = concat(function(data) {})
// data will be [1,2,3,4,5,6] in the above callback


var write = concat(function(data) {})
var a = new Uint8Array(3)
a[0] = 97; a[1] = 98; a[2] = 99

See test/ for more examples


var concat = require('concat-stream')

var writable = concat(opts={}, cb)

Return a writable stream that will fire cb(data) with all of the data that was written to the stream. Data can be written to writable as strings, Buffers, arrays of byte integers, and Uint8Arrays.

By default concat-stream will give you back the same data type as the type of the first buffer written to the stream. Use opts.encoding to set what format data should be returned as, e.g. if you if you don't want to rely on the built-in type checking or for some other reason.

  • string - get a string
  • buffer - get back a Buffer
  • array - get an array of byte integers
  • uint8array, u8, uint8 - get back a Uint8Array
  • object, get back an array of Objects

If you don't specify an encoding, and the types can't be inferred (e.g. you write things that aren't in the list above), it will try to convert concat them into a Buffer.

If nothing is written to writable then data will be an empty array [].

error handling

concat-stream does not handle errors for you, so you must handle errors on whatever streams you pipe into concat-stream. This is a general rule when programming with node.js streams: always handle errors on each and every stream. Since concat-stream is not itself a stream it does not emit errors.

We recommend using end-of-stream or pump for writing error tolerant stream code.