App.js

import React from "react";
import { useState } from "react";

function Square({ value, onSquareClick }) {
  return (
    <button className="square" onClick={onSquareClick}>
      {value}
    </button>
    );
}

export default function Board() {
  const [xIsNext, setXIsNext] = useState(true);
  const [squares, setSquares] = useState(Array(9).fill(null));

  function handleClick(i) {
    if (squares[i] || calculateWinner(squares)) {
      return;
    }
    const nextSquares = squares.slice();
    if (xIsNext) {
      nextSquares[i] = "X";
    } else {
      nextSquares[i] = "O";
    }
    setSquares(nextSquares);
    setXIsNext(!xIsNext);
  }

  const winner = calculateWinner(squares);
  let status;
  if (winner) {
    status = "Winner: " + winner;
  } else {
    status = "Next player: " + (xIsNext ? "X" : "O");
  }



  return (
    <div>
      <div className="status">{status}</div>
      <div className="board-row">
        <Square value={squares[0]} onSquareClick={() => handleClick(0)}/>
        <Square value={squares[1]} onSquareClick={() => handleClick(1)}/>
        <Square value={squares[2]} onSquareClick={() => handleClick(2)}/>
      </div>
      <div className="board-row">
        <Square value={squares[3]} onSquareClick={() => handleClick(3)}/>
        <Square value={squares[4]} onSquareClick={() => handleClick(4)}/>
        <Square value={squares[5]} onSquareClick={() => handleClick(5)}/>
      </div>
      <div className="board-row">
        <Square value={squares[6]} onSquareClick={() => handleClick(6)}/>
        <Square value={squares[7]} onSquareClick={() => handleClick(7)}/>
        <Square value={squares[8]} onSquareClick={() => handleClick(8)}/>
      </div>
    </div>
  );
}

function calculateWinner(squares) {
  const lines = [
    [0, 1, 2], 
    [3, 4, 5],
    [6, 7, 8],
    [0, 3, 6],
    [1, 4, 7],
    [2, 5, 8],
    [0, 4, 8],
    [2, 4, 6]
  ];
  for (let i = 0; i < lines.length; i++) {
    const [a, b, c] = lines[i];
    if (squares[a] && squares[a] === squares[b] && squares[a] === squares[c]) {
      return squares[a];
    }
    let j = 0;
    while (j < 9) {
      if (!squares[j]) {
        j = 10;
      }
      j = j + 1;
    }
    if (j === 9) {
      return "It's a tie!";
    }
  }
  return null;
}