Composing duplex streams

A duplex stream is a stream with a readable and writable interface. We can take a readable stream and a writeable stream and join them as a duplex stream using the duplexify module.

Let's create a folder called composing-duplex-streams, initialize it as a package, install from2, to2, and duplexify, and create an an index.js file:

$ mkdir composing-duplex-streams 
$ cd composing-duplex-streams 
$ npm init -y 
$ npm install --save from2 to2 duplexify 
$ touch index.js 

Then in our index.js file we'll write:

const from = require('from2') 
const to = require('to2') 
const duplexify = require('duplexify') 
 
const rs = from(() => { 
  rs.push(Buffer.from('Hello, World!')) 
  rs.push(null) 
}) 
 
const ws = to((data, enc, cb) => { 
  console.log(`Data written: ${data.toString()}`) 
  cb() 
}) 
 
const stream = duplexify(ws, rs) 
 
stream.pipe(stream) 

We're using the same readable and writable streams from the main recipe (rs and ws); however, we create the stream assignment by passing ws and rs to duplexify. Now instead of piping rs to ws, we can pipe stream to itself.

This can be a very useful API pattern when we want to return or export two streams that are interrelated in some way.