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This is a super thin wrapper around through2 that works like but for streams.

For when through2 is just too verbose ūüėČ

Note you will NOT be able to skip chunks. This is intended for modification only. If you want filter the stream content, use either through2 or through2-filter. This transform also does not have a flush function.

IMPORTANT: If you return null from your function, the stream will end there.

var map = require("through2-map")
var truncate = map(function (chunk) {
  return chunk.slice(0, 10)
// vs. with through2: 
var truncate = through2(function (chunk, encoding, callback) {
  this.push(chunk.slice(0, 10))
  return callback()
// Then use your map: 
// Additionally accepts `wantStrings` argument to convert buffers into strings 
var stripTags = map({wantStrings: true}, function (str) {
  // OMG don't actually use this 
  return str.replace(/<.*?>/g, "")
// Works like `` meaning you can specify a function that 
// takes up to two* arguments: fn(chunk, index) 
var spaceout = map({wantStrings: true}, function (chunk, index) {
  return (index % 2 == 0) ? chunk + "\n\n" : chunk
// vs. with through2: 
var spaceout = through2(function (chunk, encoding, callback) {
  if (this.index == undefined) this.index = 0
  var buf = (this.index++ % 2 == 0) ? Buffer.concat(chunk, new Buffer("\n\n")) : chunk
  return callback()

*Differences from

  • Cannot insert null elements into the stream without aborting.
  • No third array callback argument. That would require realizing the entire stream, which is generally counter-productive to stream operations.
  • doesn't modify the source Array, which is somewhat nonsensical when applied to streams.


require("through2-map")([options,] fn)

Create a stream.Transform instance that will call fn(chunk, index) on each stream segment.

var Tx = require("through2-map").ctor([options,] fn)

Create a reusable stream.Transform TYPE that can be called via new Tx or Tx() to create an instance.

require("through2-map").obj([options,] fn)

Create a through2-map instance that defaults to objectMode: true.

require("through2-map").objCtor([options,] fn)

Just like ctor, but with objectMode: true defaulting to true.


  • wantStrings: Automatically call chunk.toString() for the super lazy.
  • all other through2 options