BlackFridayDateByYear.js

/**
 * @param {number} year
 * @return {Date}
 */
function getBlackFridayDate(year) {
  const date = new Date(year, 10, 1); // first day of November
  const day = date.getDay();
  if (day < 4) {
    date.setDate(date.getDate() + (4 - day)); // first Thusday of November
  } else if (day > 4) {
    date.setDate(date.getDate() + 7 - (day - 4)); // first Thusday of November
  }

  date.setDate(date.getDate() + 7 * 3 + 1); // fourth Friday of November

  return date;
}