function buildDeck() {
  const suits = ['spades', 'hearts', 'diamonds', 'clubs'];
  const ranks = ['A', '2', '3', '4', '5', '6', '7', '8', '9', '10', 'J', 'Q', 'K'];
  const deck = [];
  for (let r = 0; r < ranks.length; r++) {
    for (let s = 0; s < suits.length; s++) {
      deck.push({ ranks: ranks[r], suits: suits[s], value: r + 1 });
    }
  }
  return deck;
}
console.log(buildDeck())

const compare = (firstCard, secondCard) => {
  const cardValue = firstCard.value - secondCard.value;
  return cardValue;
}
console.log(compare());

const cardValue = firstCard.value - secondCard.value;
TypeError: Cannot read property 'value' of undefined