Date object's default day

date-day.js

function createDateString(day, month, year) {
	return year + '-' + month + '-' + day;
}

function validateDate(dateStr) {
	var ourDate = new Date(dateStr);
  if(/(Invalid)/.test(ourDate)) {
	  return false;
  }
  return true;
}

var firstDate = createDateString('01', '01', '1990');
console.log(validateDate(firstDate));

var secondDate = createDateString('', '01', '1990');
console.log(validateDate(secondDate));

var thirdDate = createDateString(null, '01', '1990');
console.log(validateDate(thirdDate));

http://yournextcto.com/index.php/2016/09/30/date-objects-behaviour-when-using-yyyy-mm-dd-but-omitting-the-dd/