text: import React from 'react';
  import ReactDOM from 'react-dom';
  import './index.css';
  
    function Square(props) {
      return (
        <button className="square" onClick={props.onClick}>
          {props.value}
        </button>
      );
    }
    
    class Board extends React.Component {
      constructor(props) {
        super(props);
        this.state = {
          squares: Array(9).fill(null),
          xIsNext: true,
        };
      }
      
      handleClick(i) {
        const squares = this.state.squares.slice();
        if (calculateWinner(squares) || squares[i]) {
          return;
        }
        squares[i] = this.state.isNext? 'X' : 'O';
        this.setState({
          squares: squares,
          isNext: !this.state.isNext,
        });
      }
  
      renderSquare(i) {
        return (<Square 
          value={this.state.squares[i]}
          onClick={() => this.handleClick(i)}
        />);
      }
    
      render() {
        const winner = calculateWinner(this.state.squares);
        let status;
        if (winner) {
          status = 'Winner: ' + winner;
        } else {
          status = 'Next player: ' + (this.state.xIsNext ? 'X' : 'O');
        }
    
        return (
          <div>
            <div className="status">{status}</div>
            <div className="board-row">
              {this.renderSquare(0)}
              {this.renderSquare(1)}
              {this.renderSquare(2)}
            </div>
            <div className="board-row">
              {this.renderSquare(3)}
              {this.renderSquare(4)}
              {this.renderSquare(5)}
            </div>
            <div className="board-row">
              {this.renderSquare(6)}
              {this.renderSquare(7)}
              {this.renderSquare(8)}
            </div>
          </div>
        );
      }
    }
    
    class Game extends React.Component {
      render() {
        return (
          <div className="game">
            <div className="game-board">
              <Board />
            </div>
            <div className="game-info">
              <div>{/* status */}</div>
              <ol>{/* TODO */}</ol>
            </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];
        }
      }
      return null;
    }
    
    // ========================================
    
    ReactDOM.render(
      <Game />,
      document.getElementById('root')
    );
    