refactoring: How can I improve this as a clean code

Question

I am currently studying JS myself.
And make the pig-game which is project of the course that I watched.
I'm always curious how can I improve my code but I got no idea where do I begin.
Is there some principle that I can improve any code?
If there's a way, Could you let me know?
Thanks!
https://github.com/wonkooklee/pig-game
result: https://wonkooklee.github.io/pig-game/

below is main functions

document.querySelector('.btn-roll').addEventListener('click', function() {
  
  if (gamePlaying) {
    
    dice = Math.floor(Math.random() * 6) + 1;
    diceDOM.style.display = 'block';
    diceDOM.src = `dice-${dice}.png`;

    if (dice === 6 && lastDice === 6) {
      // Player looses score
      scores[activePlayer] = 0;
      document.getElementById(`score-${activePlayer}`).textContent = '0';
      nextPlayer();
    } else if (dice !== 1 && dice !== 6) {
      roundScore += dice;
      document.getElementById(`current-${activePlayer}`).textContent = roundScore;
      lastDice = 0;
    } else if (dice === 6) {
      roundScore += dice;
      document.getElementById(`current-${activePlayer}`).textContent = roundScore;
      lastDice = dice;
    } else {
      nextPlayer();
    }


  }

});

document.querySelector('.btn-hold').addEventListener('click', function() {
  if (gamePlaying) {
    scores[activePlayer] += roundScore;
    document.getElementById(`score-${activePlayer}`).textContent = scores[activePlayer];

    let input = document.getElementById('scoreSet').value;
    let winningScore;

    if (isNaN(input) === false) {
      winningScore = input;
    } else {
      document.getElementById('scoreSet').value = '100';
    }

    if (scores[activePlayer] >= winningScore) {

      document.getElementById(`name-${activePlayer}`).textContent = 'WINNER!';
      document.querySelector(`.player-${activePlayer}-panel`).classList.add('winner');
      document.querySelector(`.player-${activePlayer}-panel`).classList.remove('active');
      diceDOM.style.display = 'none';
      gamePlaying = false;
    } else {
      nextPlayer();
    }

  }

});

Answer 1

Martin Fowler wrote a great book "Refactoring" . Besides, Fowler has a great blog Refactoring.com , where you can find a lot of information about refactoring with examples in Javascript. I'm not strong in Javascript, but can let you some advices about your code.

1. Simplify Conditional Logic

For example like this:

if (dice === 6 && lastDice === 6) {
  // Player looses score
  scores[activePlayer] = 0;
  document.getElementById(`score-${activePlayer}`).textContent = '0';
  nextPlayer();
  return;
} 

if (dice !== 1 && dice !== 6) {
  roundScore += dice;
  document.getElementById(`current-${activePlayer}`).textContent = roundScore;
  lastDice = 0;
  return;
} 

if (dice === 6) {
  roundScore += dice;
  document.getElementById(`current-${activePlayer}`).textContent = roundScore;
  lastDice = dice;
  return;
}

nextPlayer();

2.Delete duplicated code and extract function

For example

function someFunctionName(diffRoundScore, lastDiceValue){
  roundScore += diffRoundScore;
  document.getElementById(`current-${activePlayer}`).textContent = roundScore;
  lastDice = lastDiceValue;
}

if (dice !== 1 && dice !== 6) {
  someFunctionName(dice, 0);
  return;
} 

if (dice === 6) {
  someFunctionName(dice, dice);
  return;
}

3.Change check "dice for 6" to function

function isDiceEqualsSix { return dice === 6};

if (isDiceEqualsSix && lastDice === 6) {
  // Player looses score
  scores[activePlayer] = 0;
  document.getElementById(`score-${activePlayer}`).textContent = '0';
  nextPlayer();
  return;
} 

if (dice !== 1 && !isDiceEqualsSix) {
  someFunctionName(dice, 0);
  return;
} 

if (isDiceEqualsSix) {
  someFunctionName(dice, dice);
  return;
}

4.Change "6" to a constant variable or a function

const LIMIT_SCORE = 6;

function isDiceEqualsSix { return dice === LIMIT_SCORE};

if (isDiceEqualsSix && lastDice === LIMIT_SCORE) {
  // Player looses score
  scores[activePlayer] = 0;
  document.getElementById(`score-${activePlayer}`).textContent = '0';
  nextPlayer();
  return;
} 

I hope I helped you.