let demo1 = new Promise((hahaha, rej) => {
  if (Math.random() >= 0.5) {
    console.log('Promise 1 : resolved',{ 'promise1': 'cats world' });
    hahaha({ 'promise1': 'cats world' });
  }
  else {
    let error = 'some error number/code/object'
    console.log('Promise 1 : rejected','promise1bye' + error);
    rej('promise1bye:' + error);
  }
});

// demo1.then(x => console.log(x)).catch(x => console.log(x));

let demo2 = new Promise((res, rej) => {
  if (Math.random() >= 0.5) {
    setTimeout(() => {
      console.log('Promise 2 : resolved',{ 'promise2': 'dogs world' });
      res({ 'promise2': 'dogs world' });
    }, 3000);
  }
  else {
    let error = 'some error number/code/object'
      setTimeout(() => {
      console.log('Promise 2 : rejected',{ 'promise2': 'byeerror' });
      rej({'promise2': 'byeerror'});
    }, 3000);
  }
});

// demo2.then(x => console.log(x)).catch(x => console.log(x));

let demo3 = Promise.all([demo1, demo2]).then(data => {
  console.log('Promise.all resolved',data);
}).catch(error => {
  console.log('Promise.all resolved',error);
})