node/lib/_stream_passthrough.js

// a passthrough stream.
// basically just the most minimal sort of Transform stream.
// Every written chunk gets output as-is.

'use strict';

module.exports = PassThrough;

const Transform = require('_stream_transform');
const util = require('util');
util.inherits(PassThrough, Transform);

function PassThrough(options) {
  if (!(this instanceof PassThrough))
    return new PassThrough(options);

  Transform.call(this, options);
}

PassThrough.prototype._transform = function(chunk, encoding, cb) {
  cb(null, chunk);
};