Using PassThrough streams

This sort of stream is a trivial implementation of a Transform stream, which simply passes received input bytes through to an output stream. This is useful if one doesn't require any transformation of the input data, and simply wants to easily pipe a Readable stream to a Writable stream.

PassThrough streams have benefits similar to JavaScript's anonymous functions, making it easy to assert minimal functionality without too much fuss. For example, it is not necessary to implement an abstract base class, as one does with for the _read method of a Readable stream. Consider the following use of a PassThrough stream as an event spy:

const fs = require('fs');
const stream = require('stream');
const spy = new stream.PassThrough();

spy
.on('error', (err) => console.error(err))
.on('data', function(chunk) {
    console.log(`spied data -> ${chunk}`);
})
.on('end', () => console.log('\nfinished'));

fs.createReadStream('./passthrough.txt').pipe(spy).pipe(process.stdout);

Normally a Transform or Duplex stream is what you want (where you can set up a proper implementation of _read and _write), but in certain scenarios, such as tests, it can be useful to place "watchers" on a stream.