pascua/index.js

Get Easter date for any given year

const intDiv = (a, b) => Math.trunc(a / b);

// "Meeus/Jones/butcher" algorithm
// https://en.wikipedia.org/wiki/Computus
function pascua(year = new Date().getUTCFullYear()) {
  if (year < 1583 || year > 4099) {
    throw new Error("The year should be between 1583 and 4099");
  }
  const a = year % 19;
  const b = intDiv(year, 100);
  const c = year % 100;
  const d = intDiv(b, 4);
  const e = b % 4;
  const f = intDiv(b + 8, 25);
  const g = intDiv(b - f + 1, 3);
  const h = (19 * a + b - d - g + 15) % 30;
  const i = intDiv(c, 4);
  const k = c % 4;
  const l = (32 + 2 * e + 2 * i - h - k) % 7;
  const m = intDiv(a + 11 * h + 22 * l, 451);
  const n = h + l - 7 * m + 114;
  const month = intDiv(n, 31);
  const day = 1 + (n % 31);
  return {
    year,
    month,
    day,
    toString() {
      const month = String(this.month);
      const day = String(this.day);
      return `${this.year}-${month.padStart(2, "0")}-${day.padStart(2, "0")}`;
    },
  };
}

export default pascua;