date-sort.js

const people = [
  { id: 12, name: 'Billy', dob: '1998-10-05' },
  { id: 123, name: 'Bart', dob: '1993-02-15' },
  { id: 45, name: 'Belinda', dob: '1996-01-31' },
  { id: 67, name: 'Bonnie', dob: '1998-04-09' },
  { id: 89, name: 'Brenda', dob: '1996-07-08' },
  { id: 34, name: 'Bobby', dob: '1994-09-12' },
  { id: 234, name: 'Blake', dob: '2000-01-01' },
];

const log = console.log;

// log('\n\n built-in sort method');
// log(people.sort()); // [Object object]

// log('\n\n sort by name');
// log(people.sort(byName));

// log('\n\n sort by id');
// log(people.sort(byId));

// log('\n\n sort by date');
// log(people.sort(byDate));

log('\n\n sort by birthday as it occurs during the year');
log(people.sort(byBirthday));

function byName(a, b) {
  //alphabetically by name
  if (a.name > b.name) {
    return 1;
  } else if (b.name > a.name) {
    return -1;
  } else {
    return 0;
  }
}

function byId(a, b) {
  //numerically by id
  return parseInt(a.id) - parseInt(b.id);
}

function byDate(a, b) {
  //chronologically by year, month, then day
  return new Date(a.dob).valueOf() - new Date(b.dob).valueOf(); //timestamps
}

function byBirthday(a, b) {
  //by month and then by day
  let d1 = new Date(a.dob); // 1993-02-15T00:00:00Z =>   1993-02-14T20:00:00EST
  let d2 = new Date(b.dob);
  log(d1.getDate(), d1.getUTCDate(), d1.getMonth(), d1.getUTCMonth());
  if (d1.getUTCMonth() > d2.getUTCMonth()) {
    return 1;
  } else if (d1.getUTCMonth() < d2.getUTCMonth()) {
    return -1;
  } else {
    //same month
    return d1.getUTCDate() - d2.getUTCDate();
  }
}