• Trick: Shuffle - it's a process to sort in random order :)
  

  coursa4/js/core.js

  Lines 48 to 58 in 05a7bce

  	const shuffle = (cards) => {
  	const deck = cards,
  	len = cards.length;
  	for (let i = 0; i < len; i++) {
  	const num = getRandom(i, len - 1);
  	const temp = deck[i];
  	deck[i] = deck[num];
  	deck[num] = temp;
  	}
  	return deck;
  	};

const shuffle = (list) => list.sort(() => Math.random() > 0.5 ? 1 : -1; 
// Note: that the array is sorted in place, and no copy is made.

• It's better to hardcode values intead of generating it
  

  coursa4/js/core.js

  Lines 31 to 46 in 05a7bce

  	const defCards = () => {
  	const range = ['J', 'Q', 'K', 'A'],
  	suits = [
  	'heart',
  	'diamond',
  	'spade',
  	'club'
  	],
  	cards = [];
  	for (let i = 10; i >= 2; i--)
  	range.unshift(i);
  	for (const suit of suits)
  	for (const val of range)
  	cards.push({ suit: suit, value: val });
  	return cards;
  	};

const range = ['J', 'Q', 'K', 'A'];
for (let i = 10; i >= 2; i--)
   	range.unshift(i);

can be replaced with

const range = [2, 3, 4, 5, 6, 7, 8, 9, 10, 'J', 'Q', 'K', 'A'];

 const range = [2, 3, 4, 5, 6, 7, 8, 9, 10, 'J', 'Q', 'K', 'A'];
 const suits = ['heart', 'diamond', 'spade', 'club'];
 const cards = suits.flatMap(suit => range.map(value => ({suit, value})));

Note: you can define card cost too.

• Avoid making aliases. Just use original methods
  

  coursa4/js/core.js

  Lines 60 to 61 in 05a7bce

  	const findId = (id) => document.getElementById(id);
  	const findClass = (className) => document.getElementsByClassName(className);

• Use .filter method

  coursa4/js/core.js

  Lines 114 to 123 in 05a7bce

  	const countAces = (deck) => {
  	const aces = [];
  	for (const ace of deck) {
  	if (ace.value === 'A')
  	aces.push(ace);
  	}
  	return aces;
  	};

• Looks like you want to transform array of cards into single value of score. Try to use .reduce method which is ideal for this kind of operation
  

  coursa4/js/core.js

  Lines 100 to 143 in 05a7bce

  	const countScore = (cards) => {
  	let score = 0;
  	const defVal = (checkCard) => {
  	if (typeof checkCard.value !== 'number') {
  	if (checkCard.value === 'A')
  	return 11;
  	else
  	return 10;
  	} else {
  	return checkCard.value;
  	}
  	};
  	const countAces = (deck) => {
  	const aces = [];
  	for (const ace of deck) {
  	if (ace.value === 'A')
  	aces.push(ace);
  	}
  	return aces;
  	};
  	for (const card of cards) {
  	const val = defVal(card);
  	score += val;
  	}
  	const aces = countAces(cards);
  	if (aces.length > 0) {
  	let n = 0;
  	while (score > 21 && n < aces.length) {
  	n++;
  	score -= 10;
  	}
  	}
  	return score;
  	};

• Use destructuring
  

  coursa4/js/core.js

  Line 151 in 05a7bce

  const card = result.leftInDeck.splice(0, 1)[0],

const [card, ...leftInDeckCards] = result.leftInDeck;

• Why you search for element by id if you already have this element newCard

  coursa4/js/core.js

  Line 166 in 05a7bce

  findId(newCard.id).style.transform = 'scale(1)';

• Are you sure you want to call parseCardClass method instead of hardcoding "hidden"?
  

  coursa4/js/core.js

  Line 153 in 05a7bce

  className = 'card' + parseCardClass(false, card),

• Note: You can fill id value at the init cards phase or you can add getter to your card object
  

  coursa4/js/core.js

  Line 154 in 05a7bce

  idOfCard = `card_player_${result.cards.length}`;

• Seems like a dulpicate
  

  coursa4/js/core.js

  Lines 145 to 197 in 05a7bce

  	const hit = (deck, playerCards) => {
  	const result = {
  	leftInDeck: deck,
  	cards: playerCards
  	};
  	const card = result.leftInDeck.splice(0, 1)[0],
  	newCard = document.createElement('table'),
  	className = 'card' + parseCardClass(false, card),
  	idOfCard = `card_player_${result.cards.length}`;
  	card.id = idOfCard;
  	result.cards.push(card);
  	newCard.id = idOfCard;
  	newCard.className += className;
  	newCard.innerHTML = CARD_TEMPLATE;
  	findClass('playerCards')[0].appendChild(newCard);
  	setTimeout(() => {
  	findId(newCard.id).style.transform = 'scale(1)';
  	}, 100);
  	return result;
  	};
  	const stand = (deck, dealerCards) => {
  	const result = {
  	leftInDeck: deck,
  	cards: dealerCards
  	};
  	const card = result.leftInDeck.splice(0, 1)[0],
  	newCard = document.createElement('table'),
  	className = 'card' + parseCardClass(false, card),
  	idOfCard = `card_dealer_${result.cards.length}`;
  	card.id = idOfCard;
  	result.cards.push(card);
  	newCard.id = idOfCard;
  	newCard.className += className;
  	newCard.innerHTML = CARD_TEMPLATE;
  	findClass('dealerCards')[0].appendChild(newCard);
  	setTimeout(() => {
  	findId(newCard.id).style.transform = 'scale(1)';
  	}, 100);
  	return result;
  	};

===========================================================================
To be continued ....