cardOfTheWeek.js

function isLeapYear(date) {
    var year = date.getFullYear();
    if((year & 3) != 0) return false;
    return ((year % 100) != 0 || (year % 400) == 0);
};

// https://stackoverflow.com/a/26426761
function getDOY(date) {
    var dayCount = [0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334];
    var mn = date.getMonth();
    var dn = date.getDate();
    var dayOfYear = dayCount[mn] + dn;
    if(mn > 1 && isLeapYear(date)) dayOfYear++;
    return dayOfYear;
};

function cardOfTheWeek(date) {
  const days = getDOY(date);
  const week = Math.floor(days / 7);

  if (week == 52) {
    // actually the white joker, because the black joker is rendered as an emoji
    // color: black though because the text is on a white background
    return { glyph: "🃟", text: "Jkr", color: "black"};
  } else {
    const SUITS = "♦♣♥♠"; // Big 2 instead of Poker for alternating colors
    const CARDS = ["A", "2", "3", "4", "5", "6", "7", "8", "9", "10", "J", "Q", "K"];

    const CLUBS_OFFSET = 0x1F0D1;
    const DIAMONDS_OFFSET = 0x1F0C1;
    const HEARTS_OFFSET = 0x1F0B1;
    const SPADES_OFFSET = 0x1F0A1;
    const SUIT_OFFSETS = [DIAMONDS_OFFSET, CLUBS_OFFSET, HEARTS_OFFSET, SPADES_OFFSET];

    const suitIdx = Math.floor(week / 13);
    const cardIdx = week % 13
    const glyph = String.fromCodePoint(SUIT_OFFSETS[suitIdx] + cardIdx);
    const text = CARDS[cardIdx] + SUITS[suitIdx];
    return { glyph, text, color: suitIdx % 2 ? "black" : "red" };
  }
}