const el = document.querySelector('#user');

// el.id === 'user'
// el.dataset.id === '1234567890'
// el.dataset.user === 'johndoe'
// el.dataset.dateOfBirth === ''

// set a data attribute
el.dataset.dateOfBirth = '1960-10-03';
// Result: el.dataset.dateOfBirth === '1960-10-03'

delete el.dataset.dateOfBirth;
// Result: el.dataset.dateOfBirth === undefined

if ('someDataAttr' in el.dataset === false) {
  el.dataset.someDataAttr = 'mydata';
  // Result: 'someDataAttr' in el.dataset === true
}