React チュートリアル

```
import React from 'react';
import ReactDOM from 'react-dom';
import './index.css';

class Square extends React.Component {

      constructor(){
        super();
        this.state = {
          value: null
        };
      }
      render() {
        return (
          <button className="square" onClick={() => this.props.onClick()}>
            {this.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() {
    super();
    this.state = {
      history: [{
        squares: Array(9).fill(null)
      }],
      xIsNext: true,
      stepNumber: 0
    };
  }
  handleClick(i) {
    var history = this.state.history;
    var 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
        }]),
        xIsNext: !this.state.xIsNext,
        stepNumber: history.length
      });
    }
  jumpTo(step) {
    this.setState({
      stepNumber: step,
      xIsNext: (step % 2) ? false : true,
    });
  }
  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 ?
        'Move #' + move :
        'Game start';
      return (
        <li key={move}>
          <a href="#" onClick={() => this.jumpTo(move)}>{desc}</a>
        </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('container')
);

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;
}
```