A through2 to create an Array.prototype.map analog for streams.
Latest commit 2ba6ff9 Jun 16, 2015 @brycebaril 2.0.0
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LICENSE Initial version Aug 5, 2013




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

For when through2 is just too verbose 😉

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 `Array.prototype.map` 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 Array.prototype.map:

  • 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.
  • Array.prototype.map 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