my-duplex-stream.js

const { Duplex } = require('stream');

class MyDuplexStream extends Duplex {
  constructor(bytes) {
    this.buffer = Buffer.alloc(bytes, '', 'utf-8');
    this.bytesRead = 0;
  }
  
  _read(n) {
    this.bytesRead += n;
    return this.buffer.toString('utf-8', this.bytesRead - n, this.bytesRead);
  }
  
  _write(chunk) {
    this.buffer.write(chunk);
  }
}