Node.js read file line by line and transform it

readline.js

const fs = require('fs');
const readline = require('readline');

const readFile = readline.createInterface({
  input: fs.createReadStream('file.in'),
  output: fs.createWriteStream('file.out'),
  terminal: false
});

readFile
  .on('line', transform)
  .on('close', function() {
    console.log(`Created "${this.output.path}"`);
  });

function transform(line) {
  this.output.write(`modified ${line}\n`);
}