How to pass dates to getFullYear ()?

must accept the date of men. how to convert date using getFullYear?

My code const getMenCountByYear = (users) => {

    const men = users.filter((users) => users.gender == 'male');
    const years = users.map((users) => new Date(users.birthday));
    const yer = years.getFullYear();

    return dates = (acc, year) => {
    if(_.has(year, acc)){
      acc[year] =+ 1;
    } else{ 
      acc[year] = 1;
    }
    return acc;
  };

  const userByYear = yer.reduce(dates, []);
  console.log(yer);
}

console.log(getMenCountByYear(users));
export default getMenCountByYear(users);