JSFiddle - React, Tailwind, and code Playground

HTML

<input id='btn_explore' type='button' value='Explore the Dungeon ...' />
<br />
<label id='actionOutput' >Start exploring the dungeon ...?</label>
<hr />
<div id='monsterDisplay'>
  <label id='monsterName'></label>&nbsp;
  <progress id='monsterHpValue' ></progress>&nbsp;
  <label id='monsterHp'></label>
  <label id='monsterHpMax'></label>
  <br />
  <input id='btn_hit' type='button' value='Hit it!' />
  <br />
  <label id='combatLog'></label><br />
  <label id='lootLog'></label>  
</div>

JavaScript

class Monster{
  constructor(monsterName, maxHp){
    this.monsterName = monsterName;
    this.maxHp = maxHp;
    this.currentHp = maxHp;
    this.alive = true;
  }
  
  takeDamage(damage){
    if(this.alive){
      this.currentHp = this.currentHp-damage;
      if(this.currentHp <= 0){
        this.alive = false;
      }
       else{
        this.alive = true;
      }      
      
      updateGUI(this);
    } 
  }
}

class Item{
  constructor(itemName, itemSlot, goldValue, atkValue){
    this.itemName = itemName;
    this.itemSlot = itemSlot;
    this.goldValue = goldValue;
    this.atkValue = atkValue;
  }
}

// your variables
var monsters = [];
var lootableItems = [];
var playerItems;
var activeMonster;
var activeWeapon;

function initGUI(){
	document.getElementById('btn_explore').addEventListener('click', function(){drawMonster();});
	document.getElementById('btn_hit').addEventListener('click', function(){hitMonster();});
  
  document.getElementById('monsterDisplay').style.visibility = "hidden";
}

function initMonsters(){ // function to initialize your monsters!
	monsters.push(new Monster('Goblin', 100));
	monsters.push(new Monster('Bandit', 200));
	monsters.push(new Monster('Dragon', 500));
  monsters.push(new Monster('Raphel Holzhauser', 5000));
}

function initItems(){ // function to initialize some Items!
	lootableItems.push(new Item('Stick', 'Weapon', 10, 1));
  lootableItems.push(new Item('Short Sword', 'Weapon', 100, 10));
  lootableItems.push(new Item('Fahnenstangl', 'Weapon', 30000, 500));
}

function drawMonster(){ // function to draw a monster from your monster array!
	var myMonster = monsters[Math.floor(Math.random()*monsters.length)]; // gets a random monster from the array
  
  // create a meaningful message based on the monster and its properties
  document.getElementById('actionOutput').innerHTML = "A wild " + myMonster.monsterName + " attacks you! It has " + myMonster.maxHp + " HP!"; 
  
  // update some more GUI elements
 ...