.js

const { Transform } = require('stream');
const {aStream, bStream } = require("other_file");

class Convert extends Transform {
  constructor() {
    super();
    this.aStream
     .pipe(bStream)
    bStream
     .on('data', data => this.cb(null, this.chunk));
  }
  _transform(chunk, encoding, cb) {
    // is there a better way to do this without assigining cb to this?
    this.cb = cb;
    this.chunk = chunk;
    this.aStream.write(chunk);
  }  
}