minimax.js

function bestMove() {
    // AI to make its turn
    let bestScore = -Infinity;
    let move;
    for (let i = 0; i < 3; i++) {
      for (let j = 0; j < 3; j++) {
        if (board[i][j] == '') {
            board[i][j] = ai;
            let score = minimax(board, 0, false);
            board[i][j] = '';
            if (score > bestScore) {
                bestScore = score;
                move = {i, j};
            }
        }
      }
    }
    board[move.i][move.j] = ai;
    currentPlayer = human;
}

let scores = {
    'X': 1,
    'O': -1,
    'tie': 0
}

function minimax(board, depth, isMaximizing) {
    let result = checkWinner();
    if (result !== null) {
        return scores[result];
    }

    if (isMaximizing) {
        let bestScore = -Infinity;
        for (let i = 0; i < 3; i++) {
            for (let j = 0; j < 3; j++) {
                if (board[i][j] == '') {
                    board[i][j] = ai;
                    let score = minimax(board, depth+1, false);
                    board[i][j] = '';
                    bestScore = max(score, bestScore);
                }
            }
        }
        return bestScore;
    } else {
        let bestScore = Infinity;
        for (let i = 0; i < 3; i++) {
            for (let j = 0; j < 3; j++) {
                if (board[i][j] == '') {
                    board[i][j] = human;
                    let score = minimax(board, depth+1, true);
                    board[i][j] = '';
                    bestScore = min(score, bestScore);
                }
            }
        }
        return bestScore;
    }
}