getLastSundaysOfYear.js

/*
  Day.js - format
    https://day.js.org/docs/en/display/format
*/

const getLastSundaysOfYear = (year = new Date().getFullYear()) => {
  const currentYear = dayjs().year(year);
  const lastSundaysOfYearMap = new Map();
  for(let i = 0; i < 12; i++) {
    const lastSundayOfMonth = currentYear.month(i).endOf('month').day(0);
    lastSundaysOfYearMap.set(i, lastSundayOfMonth);
  }
  return lastSundaysOfYearMap;
}

const lastSundaysOfYearMap = getLastSundaysOfYear(2025);

/*
console.log(Object.fromEntries(lastSundaysOfYearMap));

console.log(Array.from(lastSundaysOfYearMap.values()));
*/

const lastSundaysOfYearMapFormatted = lastSundaysOfYearMap.values().reduce((acc, sunday) => {
  acc[Number(sunday.format('M'))] = sunday.format('YYYY-MM-DD HH:mm:ss ZZ');
  return acc;
}, {});

console.log(JSON.stringify(lastSundaysOfYearMapFormatted, null, 2));

/*
output:

{
  "1": "2025-01-26 23:59:59 +0100",
  "2": "2025-02-23 23:59:59 +0100",
  "3": "2025-03-30 23:59:59 +0200",
  "4": "2025-04-27 23:59:59 +0200",
  "5": "2025-05-25 23:59:59 +0200",
  "6": "2025-06-29 23:59:59 +0200",
  "7": "2025-07-27 23:59:59 +0200",
  "8": "2025-08-31 23:59:59 +0200",
  "9": "2025-09-28 23:59:59 +0200",
  "10": "2025-10-26 23:59:59 +0100",
  "11": "2025-11-30 23:59:59 +0100",
  "12": "2025-12-28 23:59:59 +0100"
}
*/