app.js

let boxes = document.querySelectorAll(".box");
let resetBtn = document.querySelector("#reset-btn");
let newGameBtn = document.querySelector("#new-btn");
let msgContainer = document.querySelector(".msg-container");
let msg = document.querySelector("#msg");

let turnO = true;   //playerX, playerO

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

const resetGame = () => {
    turnO = true;
    msg.innerText = `Tic-Tac-Toe`;
    newGameBtn.style.display = "block";
    msgContainer.classList.add("msg-container");
}

const newGame = () => {
    turnO = true;
    enableBoxes();
    msg.innerText = ``;
    newGameBtn.style.display = "none";
    msgContainer.classList.remove("msg-container");
}

boxes.forEach((box) => {
    box.addEventListener("click", () => {
            if (turnO) {    
                //playerO
                box.innerText = "O";
                turnO = false;
            } else {    
                //playerX
                box.innerText = "X";
                turnO = true;
            }
            box.disabled = true;

            checkWinner();
    });
});

const disableBoxes = () => {
    for(let box of boxes) {
        box.disabled = true;
    }
};

const enableBoxes = () => {
    for(let box of boxes) {
        box.disabled = false;
        box.innerText = "";
    }
};

const showWinner = (winner) => {
    msg.innerText = `Congratulations, Winner is ${winner}`;
    msgContainer.classList.add("msg-container");
    disableBoxes();
    newGameBtn.style.display = "block";
};

const checkWinner = () => {
    for(let pattern of winPatterns) {
            let pos1Val = boxes[pattern[0]].innerText;
            let pos2Val = boxes[pattern[1]].innerText;
            let pos3Val = boxes[pattern[2]].innerText;

            if (pos1Val != "" && pos2Val != "" && pos3Val != "") {
                if (pos1Val === pos2Val && pos2Val === pos3Val) {
                    showWinner(pos1Val);
           }                    
        }
    }
};

newGameBtn.addEventListener("click", newGame);
resetBtn.addEventListener("click", resetGame);