Simple example of how stream transforms work

index.js

const { createReadStream, createWriteStream } = require('fs')
const { Transform } = require('stream')
const stream = createReadStream('./text.txt')
const writeStream = createWriteStream('./result.txt')

const transformStream = new Transform({
  transform (chunk, encoding, done) {
    let text = String(chunk)
    text = text.toLowerCase().replace(/hola/g, 'adiós')
    done(null, text)
  }
})

async function start () {
  stream
    .pipe(transformStream)
    .pipe(writeStream)
}

start()

text.txt

Hola don pepito
Hola don jose
Pasó usted por casa?
Por su casa yo pasé
Vió usted a mi abuela?
A su abuela yo la vi
Adiós don Pepito
Adiós don José