JavaScript はどのようにしてサーバー側とクライアント側の両方でファイルを読み書きできるのでしょうか?

const fs = require('fs');

// Read a file
fs.readFile('file.txt', 'utf8', (err, data) => {
  if (err) throw err;
  console.log(data);
});

// Write a file
fs.writeFile('file.txt', 'Hello world!', err => {
  if (err) throw err;
  console.log('File has been written successfully.');
});

const fileInput = document.getElementById('file-input');

fileInput.addEventListener('change', e => {
  const file = e.target.files[0];

  // Read a file
  const reader = new FileReader();
  reader.onload = () => {
    console.log(reader.result);
  };
  reader.readAsText(file);

  // Write a file (client-side)
  const filename = 'file.txt';
  const fileContent = 'Hello world!';

  const blob = new Blob([fileContent], { type: 'text/plain' });
  const url = URL.createObjectURL(blob);

  const a = document.createElement('a');
  a.href = url;
  a.download = filename;
  a.click();
});