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 {
      renderSquare(i) {
        return (
          <Square
            value={this.props.squares[i]}
            onClick={() => this.props.onClick(i)}
          />
        );
      }
    
      render() {
        return (
          <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 {
      constructor(props) {
        super(props);
        this.state = {
          history: [
            {
              squares: Array(9).fill(null)
            }
          ],
          stepNumber: 0,
          xIsNext: true
        };
      }
    
      handleClick(i) {
        const history = this.state.history.slice(0, this.state.stepNumber + 1);
        const current = history[history.length - 1];
        const squares = current.squares.slice();
        if (calculateWinner(squares) || squares[i]) {
          return;
        }
        squares[i] = this.state.xIsNext ? "X" : "O";
        this.setState({
          history: history.concat([
            {
              squares: squares
            }
          ]),
          stepNumber: history.length,
          xIsNext: !this.state.xIsNext
        });
      }
    
      jumpTo(step) {
        this.setState({
          stepNumber: step,
          xIsNext: (step % 2) === 0
        });
      }
    
      render() {
        const history = this.state.history;
        const current = history[this.state.stepNumber];
        const winner = calculateWinner(current.squares);
    
        const moves = history.map((step, move) => {
          const desc = move ?
            'Go to move #' + move :
            'Go to game start';
          return (
            <li key={move}>
              <button onClick={() => this.jumpTo(move)}>{desc}</button>
            </li>
          );
        });
    
        let status;
        if (winner) {
          status = "Winner: " + winner;
        } else {
          status = "Next player: " + (this.state.xIsNext ? "X" : "O");
        }
    
        return (
          <div className="game">
            <div className="game-board">
              <Board
                squares={current.squares}
                onClick={i => this.handleClick(i)}
              />
            </div>
            <div className="game-info">
              <div>{status}</div>
              <ol>{moves}</ol>
            </div>
          </div>
        );
      }
    }
    
    // ========================================
    
    ReactDOM.render(<Game />, document.getElementById("root"));
    
    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;
    }