TicTacToe/src/App.js

import './App.css';
import { useState } from 'react';

function Square({value, onSquareClick}) {

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

function App() {

  const [squares, setSquares] = useState(Array(9).fill(null));
  
  function handleClick(i) {
    const nextSquares = squares.slice();
    nextSquares[i] = "X";
    setSquares(nextSquares);
  }

  return (
  <>
  <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>
  </>
  );
}

export default App;