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HTML 

              
                <header>
    <h1>Tic-tac-toe</h1>
    <p><span id="player-turn"></span>'s turn</p>
  </header>
  <main id="gameboard">
    <div class="line r r-1"></div>
    <div class="line r r-2"></div>
    <div class="line c c-1"></div>
    <div class="line c c-2"></div>
  </main>
  <div id="num-of-players">
    <p>One Player</p>
    <div id="toggle-switch">
      <div id="toggle-btn"></div>
    </div>
    <p>Two Player</p>
  </div>
  <div id="gameover-msg">
    <h3 id="winner"></h3>
    <button id="reset-btn">New Game</button>
  </div>
!

CSS 

              
                @import url('https://fonts.googleapis.com/css2?family=Montserrat:wght@400;500;600&display=swap');

@mixin mainFlex {
  display: flex;
  justify-content: center;
  align-items: center;
}

$onyx: #393E41;
$ivory: #F6F7EB;
$green: #16b550;

* {
  margin: 0;
  padding: 0;
}

body {
  @include mainFlex();
  flex-direction: column;
  font-family: 'Montserrat', sans-serif;
  color: $ivory;
  height: 100vh;
  background: $onyx;
  position: relative;
}

header {
  text-align: center;
  margin-bottom: 30px;
  h1 {
    margin-bottom: 20px;
  }
  #player-turn {
    font-weight: 600;
  }
}

#gameboard {
  display: flex;
  flex-wrap: wrap;
  width: 300px;
  height: 300px;
  position: relative;
  .tile {
    @include mainFlex();
    font-size: 1.8rem;
    font-weight: 600;
    width: 100px;
    height: 100px;
    box-sizing: border-box;
    &:hover {
      cursor: pointer;
    }
  }
  .line {
    position: absolute;
    z-index: 1;
    background: $ivory;
    border-radius: 2px;
  }
  .r {
    width: 300px;
    height: 4px;
  }
  .r-1 {
    top: 98px;
  }
  .r-2 {
    bottom: 98px;
  }
  .c {
    width: 4px;
    height: 300px;
  }
  .c-1 {
    left: 98px;
  }
  .c-2 {
    right: 98px;
  }
}

#num-of-players {
  @include mainFlex();
  margin-top: 30px;
  #toggle-switch {
    margin: 0 10px 0 10px;
    width: 50px;
    height: 25px;
    border-radius: 100px;
    border: 2px solid $ivory;
    #toggle-btn {
      float: left;
      width: 25px;
      height: 25px;
      border-radius: 50%;
      background: $ivory;
      &:hover {
        cursor: pointer;
      }
    }
  }
}


#gameover-msg {
  position: absolute;
  z-index: 2;
  text-align: center;
  color: $onyx;
  width: 200px;
  height: 150px;
  background: $ivory;
  border-radius: 8px;
  box-shadow: 0 0 5px black;
  display: none;
  h3 {
    margin-top: 35px;
    margin-bottom: 20px;
  }
  button {
    font-family: 'Montserrat', sans-serif;
    font-weight: 500;
    color: $ivory;
    margin: auto;
    padding: 8px;
    background: $green;
    border-radius: 4px;
    border: none;
    &:hover {
      cursor: pointer;
    }
  }
}
!

JS 

              
                document.addEventListener('DOMContentLoaded', () => {

  const gameboard = document.getElementById('gameboard');
  const gameOverMsg = document.getElementById('gameover-msg');
  const winner = document.getElementById('winner');
  const resetBtn = document.getElementById('reset-btn');
  const playerToggleBtn = document.getElementById('toggle-btn');
  const playerTurn = document.getElementById('player-turn');
  playerTurn.textContent = 'X';

  const $ivory = '#F6F7EB';
  const $green = '#16b550';

  let tilesArr = [
    0, 1, 2,
    3, 4, 5,
    6, 7, 8
  ];

  let isOnePlayer = true;
  let isComputerTurn = false;
  let isXTurn = true;
  let isGameOver = false;
  let validPlays = 0;
  let winningCombo;

  const addTiles = () => {
    tilesArr.forEach((index) => {
      const tile = document.createElement('div');
      tile.id = index;
      tile.classList.add('tile');
      tile.addEventListener('click', () => handleClick(tile));
      gameboard.appendChild(tile);
    });
  }

  addTiles();

  const tiles = document.querySelectorAll('.tile');

  const handleClick = (tile) => {
    if (isOnePlayer === true) {
      if (isXTurn && tile.textContent === '') {
        markX(tile);
        isComputerTurn = true;
        !isGameOver ? 
        setTimeout(() => computerMarkO(), 1000) :
        false;
      }
    }
    if (isOnePlayer === false) {
      if (isXTurn && tile.textContent === '') {
        markX(tile);
      } else if (!isXTurn && tile.textContent === '') {
        markO(tile);
      }
    }
  }

  const markX = (tile) => {
    tile.textContent = 'X';
    tilesArr[tile.id] = 'X';
    validPlays++;
    isXTurn = !isXTurn;
    isXTurn ? playerTurn.textContent = 'X' : playerTurn.textContent = 'O';
    checkForWin();
    validPlays === 9 ? displayMessage() : false;
  }

  const markO = (tile) => {
    tile.textContent = 'O';
    tilesArr[tile.id] = 'O';
    validPlays++;
    isXTurn = !isXTurn;
    isXTurn ? playerTurn.textContent = 'X' : playerTurn.textContent = 'O';
    checkForWin();
    validPlays === 9 ? displayMessage() : false;
  }

  const getRandomTile = () => {
    const availTilesArr = [];
    for (let i = 0; i < tilesArr.length; i++) {
      if (tilesArr[i] !== 'X' && tilesArr[i] !== 'O') {
        availTilesArr.push(i);
      }
    }
    const randomIndex = Math.floor(Math.random() * availTilesArr.length);
    return availTilesArr[randomIndex];
  }

  const computerMarkO = () => {
    if (isComputerTurn) {
      let randomID = getRandomTile();
      let randomTile = document.getElementById(randomID);
      randomTile.textContent = 'O';
      tilesArr[randomID] = 'O';
      validPlays++;
      isXTurn = !isXTurn;
      isXTurn ? playerTurn.textContent = 'X' : playerTurn.textContent = 'O';
      checkForWin();
      validPlays === 9 ? displayMessage() : false;
    }
    isComputerTurn = false;
  }

  const checkForWin = () => {
    // winning combos
    // [0,1,2], [3,4,5], [6,7,8], [0,3,6], 
    // [1,4,7], [2,5,8], [0,4,8], [2,4,6]
    tilesArr[0] === tilesArr[1] &&
    tilesArr[1] === tilesArr[2] &&
    tilesArr[0] === tilesArr[2] ?
    gameOver(0) :
    false;

    tilesArr[3] === tilesArr[4] &&
    tilesArr[4] === tilesArr[5] &&
    tilesArr[3] === tilesArr[5] ?
    gameOver(1) :
    false;

    tilesArr[6] === tilesArr[7] &&
    tilesArr[7] === tilesArr[8] &&
    tilesArr[6] === tilesArr[8] ?
    gameOver(2) :
    false;

    tilesArr[0] === tilesArr[3] &&
    tilesArr[3] === tilesArr[6] &&
    tilesArr[0] === tilesArr[6] ?
    gameOver(3) :
    false;

    tilesArr[1] === tilesArr[4] &&
    tilesArr[4] === tilesArr[7] &&
    tilesArr[1] === tilesArr[7] ?
    gameOver(4) :
    false;

    tilesArr[2] === tilesArr[5] &&
    tilesArr[5] === tilesArr[8] &&
    tilesArr[2] === tilesArr[8] ?
    gameOver(5) :
    false;

    tilesArr[0] === tilesArr[4] &&
    tilesArr[4] === tilesArr[8] &&
    tilesArr[0] === tilesArr[8] ?
    gameOver(6) :
    false;

    tilesArr[2] === tilesArr[4] &&
    tilesArr[4] === tilesArr[6] &&
    tilesArr[2] === tilesArr[6] ?
    gameOver(7) :
    false;

  }

  const gameOver = (val) => {
    isGameOver = true;
    winningCombo = val;
    switch (winningCombo) {
      case 0:
        tiles[0].style.color = $green;
        tiles[1].style.color = $green;
        tiles[2].style.color = $green;
        break;
      case 1:
        tiles[3].style.color = $green;
        tiles[4].style.color = $green;
        tiles[5].style.color = $green;
        break;
      case 2:
        tiles[6].style.color = $green;
        tiles[7].style.color = $green;
        tiles[8].style.color = $green;
        break;
      case 3:
        tiles[0].style.color = $green;
        tiles[3].style.color = $green;
        tiles[6].style.color = $green;
        break;
      case 4: 
        tiles[1].style.color = $green;
        tiles[4].style.color = $green;
        tiles[7].style.color = $green;
        break;
      case 5:
        tiles[2].style.color = $green;
        tiles[5].style.color = $green;
        tiles[8].style.color = $green;
        break;
      case 6:
        tiles[0].style.color = $green;
        tiles[4].style.color = $green;
        tiles[8].style.color = $green;
        break;
      case 7:
        tiles[2].style.color = $green;
        tiles[4].style.color = $green;
        tiles[6].style.color = $green;
    }
    displayMessage();
  }

  const displayMessage = () => {
    if (isGameOver) {
      setTimeout(() => {
        gameOverMsg.style.display = 'block';
        playerTurn.textContent === 'X' ? 
        winner.textContent = 'Player O Wins!' : 
        winner.textContent = 'Player X Wins!';
      }, 1000);
    } 
    if (!isGameOver) {
      gameOverMsg.style.display = 'block';
      winner.textContent = 'Tie Game!'
    }
  }

  const reset = () => {
    tiles.forEach((tile) => {
      tile.textContent = '';
      tile.style.color = $ivory;
    });
    tilesArr = [
      0, 1, 2,
      3, 4, 5,
      6, 7, 8
    ];
    winningCombo = '';
    isXTurn = true;
    isGameOver = false;
    validPlays = 0;
    playerTurn.textContent = 'X';
    gameOverMsg.style.display = 'none';
  }

  const handleNumPlayers = () => {
    isOnePlayer = !isOnePlayer;
    isOnePlayer ? 
    playerToggleBtn.style.float = 'left' : 
    playerToggleBtn.style.float = 'right';
  }

  resetBtn.addEventListener('click', () => reset());
  playerToggleBtn.addEventListener('click', () => handleNumPlayers());
});
!
999px

Console