Node.js split stream in two streams.

node-js-split-stream.js

// source code from https://stackoverflow.com/a/19561718/713573
const fs = require("fs");
const {Transform} = require('stream');
const split = require('split');

const readStream = fs.createReadStream('./numbers');
const evenWriteStream = fs.createWriteStream('./evens');
const oddWriteStream = fs.createWriteStream('./odds');

class Even extends Transform {
  _transform(chunk, encoding, cb) {
    setTimeout(() => {
      const val = parseInt(chunk.toString());
      if (val % 2 === 0) cb(null, val.toString() + '\n');
      else cb();
    }, Math.random() * 10);
  }
}

class Odd extends Transform {
  _transform(chunk, encoding, cb) {
    setTimeout(() => {
      const val = parseInt(chunk.toString());
      if (val % 2 !== 0) cb(null, val.toString() + '\n');
      else cb();
    }, Math.random() * 10);
  }
}

const even = new Even();
const odd = new Odd();

readStream.pipe(split()).pipe(even).pipe(evenWriteStream);
readStream.pipe(split()).pipe(odd).pipe(oddWriteStream);