I'm following the tic tac toe tutorial.
我现在想添加一个功能，如果有一个平局（没有人赢得），然后它会更新消息“有一个平局”。看到完整的代码片段如下：

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),
      }],
      xIsNext: true,
      stepNumber: 0,
    };
  }

  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;
    } elseif (winner === null) {
      status = 'There has been a draw'
    }
    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>
    );
  }
}

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')
);

我尝试在Game类中实现这一点。在实际检查赢家的代码中--如果我理解正确的话--如果没有满足任何条件，它将返回null值。相关代码如下：

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;
}

我现在要做的是，在引用上述函数的代码中，显示一条消息，告诉我们谁赢了，我设置一个elseif，看看函数是否抛出null，如果抛出null，显示一条消息，告诉我们发生了平局。代码如下所示：

let status;
    if (winner) {
      status = 'Winner: ' + winner;
    } elseif (winner === null) {
      status = 'There has been a draw'
    }
    else {
      status = 'Next player: ' + (this.state.xIsNext ? 'X' : 'O');
    }

calculateWinner函数中return null;的用途是什么？如何才能最好地检查此应用程序上下文中的平局？有没有更好的方法？
问题代码：https://codepen.io/anon/pen/OOQxmw?editors=0010