double-characters.js

const fs = require('fs')
const {Transform} = require('stream')

class DoublingStream extends Transform {
  constructor() {
    // Process chunks as strings (instead of byte arrays)
    super({decodeStrings: false})
  }

  // Specifies how to transform each chunk.
  // `callback` must be called when the chunk has been processed.
  _transform(chunk, encoding, callback) {
    // For each character in the chunk, emit it twice to the output
    for (const character of chunk) {
      this.push(character, encoding)
      this.push(character, encoding)
    }
    callback() // indicate we're done processing chunk
  }
}

// Read file.txt into string chunks
fs.createReadStream('file.txt', 'utf8')
  // Transform each chunk with a DoublingStream.
  // Note that pipe() returns the destination stream.
  .pipe(new DoublingStream())
  // Pipe the output of the DoublingStream to doubled.txt
  .pipe(fs.createWriteStream('doubled.txt'))