let inputDate = document.getElementById("input-date");

let firstDayMonth = document.getElementById("first-day");

let weekDay = document.getElementById("week-day");

​

inputDate.addEventListener("change", function(event) {

  const day = handleDateValue(this.value);

  const firstDay = setFirstDayFromMonth(day);

  firstDayMonth.value = firstDay.join("-");

  weekDay.value = getDayOfWeek(day);

});

​

function handleDateValue(dateValue) {

  return dateValue.split("-"); 

}

​

function setFirstDayFromMonth(day) {

  const positionDay = 2;

  day[positionDay] = "01";

  return day;

}

​

function getDayOfWeek(day) {

  const dayWeek = new Date(day[0], day[1]-1, day[2]).getDay();

  return ['DOM','SEG', 'TER', 'QUA', 'QUI', 'SEX', 'SÁB'][dayWeek];

}

​

​