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Apply a through stream to the output side of another through stream, preserving the first stream as input in the returned stream.

build status

browser support


var through = require('through');
var throughout = require('throughput');
var doubler = through(function (buf) {
    var dbuf = new Buffer(buf.length * 2);
    for (var i = 0; i < dbuf.length; i++) {
        dbuf[i] = buf[Math.floor(i/2)];
var counter = through(
    function (buf) {
        this._count = (this._count || 0) + buf.length;
    function () {
        this.queue(this._count + '\n');
    .pipe(throughout(doubler, counter))


If you want to transform just the output side of a through stream, this is actually somewhat difficult and it's not immediately obvious why something like:

var duplexer = require('duplexer');
function filterOutput (a, b) {
    return duplexer(a, b);

shouldn't just work. The worst part is that it does work, until you try to pipe the results to a slow endpoint that will throttle with pause and resume.

The problem is that both duplexer and .pipe() will handle pause and resume in a mutually exclusive way that will cause the stream to hang indefinitely if a slow downstream consumer ever triggers a pause.

If you try to use event-stream's .pipeline() function you will get the same problem.


var throughout = require('throughout');

var c = throughout(a, b)

Pipe the output of a through stream a into the input of a through stream b, returning a new stream c that writes its input to a and forwards the output from b.


With npm do:

npm install throughout