scriptai.js

var origBoard;
var computerAI = false; /* computer ai is disabled now */
const humanPlayer = 'O';
const computerPlayer = 'X';
const winCombinations = [
    [0, 1, 2],
    [3, 4, 5],
    [6, 7, 8],
    [0, 3, 6],
    [1, 4, 7],
    [2, 5, 8],
    [0, 4, 8],
    [6, 4, 2]
]

const cells = document.querySelectorAll('.cell');
startGame({ai:false});

function startGame({ai}) {
    computerAI = ai
    document.querySelector(".endgame").style.display = "none";
    origBoard = Array.from(Array(9).keys());
    for (var i = 0; i < cells.length; i++) {
        cells[i].innerText = '';
        cells[i].style.removeProperty('background-color');
        cells[i].addEventListener('click', turnClick, false);
    }
}

function turnClick(square) {
    if (typeof origBoard[square.target.id] == 'number') {
        turn(square.target.id, humanPlayer)
        if (!checkWin(origBoard, humanPlayer) && !checkTie()) turn(bestSpot(), computerPlayer);
    }
}

function turn(squareId, player) {
    origBoard[squareId] = player;
    document.getElementById(squareId).innerText = player;
    let gameWon = checkWin(origBoard, player)
    if (gameWon) gameOver(gameWon)
}

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 winCombinations.entries()) {
        if (win.every(elem => plays.indexOf(elem) > -1)) {
            gameWon = { index: index, player: player };
            break;
        }
    }
    return gameWon;
}

function gameOver(gameWon) {
    for (let index of winCombinations[gameWon.index]) {
        document.getElementById(index).style.backgroundColor =
            gameWon.player == humanPlayer ? "#118AB2" : "#EF476F"; // blue : pink
    }
    for (var i = 0; i < cells.length; i++) {
        cells[i].removeEventListener('click', turnClick, false);
    }
    declareWinner(gameWon.player == humanPlayer ? "You win!" : "You lose.");
}

function declareWinner(who) {
    document.querySelector(".endgame").style.display = "block";
    document.querySelector(".endgame .text").innerText = who;
}

function emptySquares() {
    return origBoard.filter(s => typeof s == 'number');
}

function bestSpot() {
    if (computerAI)
        return minimax(origBoard, computerPlayer).index;
    else {
        // peek random desicion if computer ai is disabled
        let emptys = emptySquares()
        let index  = (Math.random() * 1000000).toFixed() % emptys.length
        return emptys[index]
    }
}

function checkTie() {
    if (emptySquares().length == 0) {
        for (var i = 0; i < cells.length; i++) {
            cells[i].style.backgroundColor = "#06D6A0"; // nice green
            cells[i].removeEventListener('click', turnClick, false);
        }
        declareWinner("Tie Game!")
        return true;
    }
    return false;
}

function minimax(newBoard, player) {
    var availSpots = emptySquares();

    if (checkWin(newBoard, humanPlayer)) {
        return { score: -10 };
    } else if (checkWin(newBoard, computerPlayer)) {
        return { score: 10 };
    } else if (availSpots.length === 0) {
        return { score: 0 };
    }
    var moves = [];
    for (var i = 0; i < availSpots.length; i++) {
        var move = {};
        move.index = newBoard[availSpots[i]];
        newBoard[availSpots[i]] = player;

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

        newBoard[availSpots[i]] = move.index;

        moves.push(move);
    }

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

    return moves[bestMove];
}