JS tool problem(play yourself) - reduced black jack

JS tool problem 2 about Markov decision process.

by Tinytsunami

HTML

<div id="demo">
  <pre></pre>
  <button>Draw</button>
  <button>Pass</button>
</div>

CSS

body {
  color: #ffffff;
  background: #20262e;
  font-family: monospace, sans-serif;
}

#demo input {
  width: 120px;
  color: #ffffff;
  background: #20262e;
  border: none;
  border-bottom: solid 1px #ffffff;
  outline: none;
}

#demo input {
  margin-right: 5px;
}

#demo button {
  cursor: pointer;
  color: #ffffff;
  background: #20262e;
  border: 1px solid #ffffff;
  outline: none;
}

#demo button:hover {
  color: #20262e;
  background: #ffffff;
}

JavaScript

(function() {
  /* HTML DOM */
  let demo = document.getElementById("demo");
  let message = demo.getElementsByTagName("pre")[0];
  let draw = demo.getElementsByTagName("button")[0];
  let pass = demo.getElementsByTagName("button")[1];

  /* constants */
  let ACTOR = {
    PLAYER: 0,
    COMPUTER: 1
  };
  let turn = ACTOR.PLAYER;

  /* variables */
  let player_card = [];
  let computer_card = [];
  let player_pass = false;
  let game_endding = false;

  /* card sum */
  let sum = function(cards) {
    return cards.length > 0 ? cards.reduce((p, c) => p + c) : 0;
  };

  /* game initialize */
  let initialize = function() {
    player_card = [];
    computer_card = [];
    player_pass = false;
    game_endding = false;
  };

  /* check winner */
  let check = function(a, b) {
    if (a > 10 && b > 10) {
      return 2;
    } else if (a > 10 && b <= 10) {
      return ACTOR.COMPUTER;
    } else if (a <= 10 && b > 10) {
      return ACTOR.PLAYER;
    } else {
      if (player_pass) {
        if (a < b) {
          return ACTOR.COMPUTER;
        } else if (a > b) {
          return ACTOR.PLAYER;
        } else if (a == b) {
          return 2;
        }
      }
    }
    return -1; //not endding
  };

  /* game main-loop */
  let next_turn = function() {
    let a = sum(player_card);
    let b = sum(computer_card);
    message.innerHTML = `玩家的牌面是：`;
    player_card.forEach((card, index) => {
      message.innerHTML += `${card}${index != player_card.length - 1 ? "," : ""}`;
    });
    message.innerHTML += ` (${a})`;
    message.innerHTML += `\n電腦的牌面是：`;
    computer_card.forEach((card, index) => {
      message.innerHTML += `${card}${index != computer_card.length - 1 ? "," : ""}`;
    });
    message.innerHTML += ` (${b})`;
    let winner = check(a, b);
    switch (winner) {
      case ACTOR.PLAYER:
        message.innerHTML += `\n玩家贏了，按 Draw 重新開始`;
        break;
      case ACTOR.COMPUTER:
        message.innerHTML += `\n電腦贏了，按 Draw 重新開始`;
        break;
     ...