moment-biz/lib/easter.js

Handle business days & weekends over multiple years

'use strict';

const moment = require('moment');

/*
  Easter Date Function for JavaScript implemented by Furgelnod (http://furgelnod.com)
  Using algorithm published at The Date of Easter (on aa.usno.navy.mil, Oct 2007)
  (https://web.archive.org/web/20071015045929/http://aa.usno.navy.mil/faq/docs/easter.php)
  The algorithm is credited to J.-M. Oudin (1940) and is reprinted in the
  Explanatory Supplement to the Astronomical Almanac, ed. P. K. Seidelmann (1992).
  See Chapter 12, "Calendars", by L. E. Doggett.
*/
// Takes a given year (y) then returns a moment object of Easter Sunday
const easter = module.exports = moment.easter = function easter(y = moment().year()) {
  if (!easter.cache)
    easter.cache = {};

  if (isNaN(+y))
    throw new TypeError('Value must be a number.');
  else if (y > 275760 || y < -271820)
    throw new RangeError('Value must be between -271820 and 275760 due to technical limitations of Date constructor.');

  y = Math.floor(y);

  if (y in easter.cache)
    return moment(easter.cache[y]);

  var c = Math.floor(y / 100);
  var n = y - 19 * Math.floor(y / 19);
  var k = Math.floor((c - 17) / 25);
  var i = c - Math.floor(c / 4) - Math.floor((c - k) / 3) + 19 * n + 15;
  i -= 30 * Math.floor(i / 30);
  i -= Math.floor(i / 28) * (1 - Math.floor(i / 28) * Math.floor(29 / (i + 1)) * Math.floor((21 - n) / 11));
  var j = y + Math.floor(y / 4) + i + 2 - c + Math.floor(c / 4);
  j -= 7 * Math.floor(j / 7);
  var l = i - j;
  var m = 3 + Math.floor((l + 40) / 44);
  var d = l + 28 - 31 * Math.floor(m / 4);

  easter.cache[y] = moment({year: y, month: m - 1, day: d});

  return moment(easter.cache[y]);
};

moment.fn.isEaster = function() {
  return moment.easter(this.year()).dayOfYear() === this.dayOfYear();
};