A through2 wrapper to for simple stream.PassThrough spies.
Switch branches/tags
Latest commit 9779ae8 Jun 17, 2015 @brycebaril brycebaril 2.0.0
Failed to load latest commit information.
test Add obj, objCtor Jun 20, 2014
.gitignore update deps, swap concat-stream for terminus due to breaking changes Jan 31, 2014
LICENSE Initial version Aug 7, 2013
README.md Add obj, objCtor Jun 20, 2014
index.js Add obj, objCtor Jun 20, 2014
package.json 2.0.0 Jun 17, 2015




This is a super thin wrapper around through2 for creating simple stream.PassThrough spies.

Saves you a tiny bit of boilerplate compared to through2 for writing stream spies.

Note you will NOT be able to do anything but spy and abort the stream pipeline. To do any filtering or transformations you should consider through2 through2-filter or through2-map.

Pass a function to run as each chunk goes through your stream pipeline. Return an Error to abort the pipeline.

var spy = require("through2-spy")

var count = 0
var countChunks = spy(function (chunk) {

// vs. with through2:
var countChunks = through2(function (chunk, encoding, callback) {
  return callback()

// Then use your spy:

// Additionally accepts `wantStrings` argument to conver buffers into strings
var nsaregex = /(open source)|(foss)|(node\.js)/i
var prizm = spy({wantStrings: true}, function (str) {
  var wiretap = str.match(nsaregex)
  if (wiretap) this.emit("OMGTERRIST", wiretap[0], str)

prizm.on("OMGTERRIST", sendDrone(/* ... */))


// Return an Error to abort the pipeline
var Meter = spy.ctor({maxBytes: 1024, bytes: 0}, function (chunk) {
  this.options.bytes += chunk.length
  if (this.options.bytes >= this.options.maxBytes) return new Error("Over 1024 byte limit!")

var meter = new Meter()


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

Create a through2-spy instance that will call fn(chunk) and then silently pass through data downstream.

require("through2-spy").ctor([options], fn)

Create a through2-spy Type that can be instantiated via new Type() or Type() to create reusable spies.

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

Create a through2-spy that defaults to objectMode = true.

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

Create a through2-spy Type that defaults to objectMode = true.


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