writing files node.js

index.js

const fs = require('fs');

const options = { encoding: 'utf-8' };

const writeStream = fs.createWriteStream('out2.txt', options);
const buffers = [];
let handled = 0;

function handler(error, buffer) {
  if (error) {
    console.error(error);
    return;
  }

  handled += 1; 

  fs.writeFile('out1.txt', buffer, options, () => {
    console.log('wrote to out1.txt');
  });

  writeStream.write(buffer, () => {
    console.log('wrote to out2.txt\'s write stream');
  });

  buffers.push(buffer);

  if (handled === 2) {
    writeStream.close(() => {
      console.log('closed file');
    });

    fs.writeFile('out3.txt', buffers.join(''), options, () => {
      console.log('wrote to out3.txt');
    });
  }
}

fs.readFile('test1.txt', options, handler);
fs.readFile('test2.txt', options, handler);

out1.txt

World 

out2.txt

Hello World

out3.txt

Hello World

test1.txt

Hello 

test2.txt

World