request.js

const getTodos = (resource) => {
  return new Promise((resolve, reject) => {
    const request = new XMLHttpRequest();

    request.addEventListener('readystatechange', () => {
      if(request.readyState === 4 && request.status === 200){
        const data = JSON.parse(request.responseText);
        resolve(data);
      }
      else if(request.readyState === 4) {
        reject('Could not fetch data');
      }
    });
    
    request.open('GET', resource);
    request.send();
  });
};

getTodos('todos/jaid.json').then(data => {
  console.log('Promise 1 resolved:', data);
  return getTodos('todos/hugo.json');
}).then(data => {
  console.log('Promise 2 resolved:', data);
  return getTodos('todos/joseph.json');
}).then(data => {
  console.log('Promise 3 resolved:', data);
}).catch(err => {
  console.log('Promise rejected:', err);
});