first_day_of_month.js

// const Sunday = 0;
// const Monday = 1;
const TUESDAY = 2;
// const Wednesday = 3;
// const Thursday = 4;
// const Friday = 5;
// const Saturday = 6;

const firstDayOfMonth = (year, month, weekDay) => {
  const DAYS_IN_WEEK = 7;
  const day = new Date(year, month, 0).getDay();
  const shift = (DAYS_IN_WEEK + weekDay - day) % DAYS_IN_WEEK;
  const date = new Date(year, month, shift);
  return date;
};

console.log('luty 2021', firstDayOfMonth(2021, 1, TUESDAY).toString());
console.log('styczeń 2021', firstDayOfMonth(2021, 0, TUESDAY).toString());