script.js

const cells = document.querySelectorAll('.cell');
const restartButton = document.getElementById('restart');
let board = ['', '', '', '', '', '', '', '', ''];
const humanPlayer = 'O';
const aiPlayer = 'X';

const winCombos = [
    [0, 1, 2],
    [3, 4, 5],
    [6, 7, 8],
    [0, 3, 6],
    [1, 4, 7],
    [2, 5, 8],
    [0, 4, 8],
    [2, 4, 6]
];

cells.forEach(cell => {
    cell.addEventListener('click', handleCellClick, { once: true });
});

restartButton.addEventListener('click', restartGame);

function handleCellClick(e) {
    const index = e.target.dataset.index;
    if (board[index] === '') {
        makeMove(index, humanPlayer);
        if (!checkWin(board, humanPlayer) && !checkTie()) {
            makeMove(bestMove(), aiPlayer);
        }
    }
}

function makeMove(index, player) {
    board[index] = player;
    document.querySelector(`[data-index='${index}']`).textContent = player;
    if (checkWin(board, player)) endGame(player);
}

function checkWin(board, player) {
    let plays = board.reduce((a, e, i) => (e === player) ? a.concat(i) : a, []);
    let gameWon = null;
    for (let [index, win] of winCombos.entries()) {
        if (win.every(elem => plays.indexOf(elem) > -1)) {
            gameWon = { index: index, player: player };
            break;
        }
    }
    return gameWon;
}

function endGame(player) {
    cells.forEach(cell => cell.removeEventListener('click', handleCellClick));
    alert(player === humanPlayer ? "You win!" : "You lose.");
}

function checkTie() {
    if (board.every(cell => cell !== '')) {
        alert("It's a tie!");
        return true;
    }
    return false;
}

function bestMove() {
    return minimax(board, aiPlayer).index;
}

function minimax(newBoard, player) {
    const availSpots = newBoard.filter(s => s === '');

    if (checkWin(newBoard, humanPlayer)) {
        return { score: -10 };
    } else if (checkWin(newBoard, aiPlayer)) {
        return { score: 10 };
    } else if (availSpots.length === 0) {
        return { score: 0 };
    }

    const moves = [];
    for (let i = 0; i < availSpots.length; i++) {
        const move = {};
        move.index = newBoard.indexOf(availSpots[i]);
        newBoard[move.index] = player;

        if (player === aiPlayer) {
            const result = minimax(newBoard, humanPlayer);
            move.score = result.score;
        } else {
            const result = minimax(newBoard, aiPlayer);
            move.score = result.score;
        }

        newBoard[move.index] = '';
        moves.push(move);
    }

    let bestMove;
    if (player === aiPlayer) {
        let bestScore = -10000;
        for (let i = 0; i < moves.length; i++) {
            if (moves[i].score > bestScore) {
                bestScore = moves[i].score;
                bestMove = i;
            }
        }
    } else {
        let bestScore = 10000;
        for (let i = 0; i < moves.length; i++) {
            if (moves[i].score < bestScore) {
                bestScore = moves[i].score;
                bestMove = i;
            }
        }
    }

    return moves[bestMove];
}

function restartGame() {
    board = ['', '', '', '', '', '', '', '', ''];
    cells.forEach(cell => {
        cell.textContent = '';
        cell.addEventListener('click', handleCellClick, { once: true });
    });
}