index.js

import React from 'react';
import ReactDOM from 'react-dom';
import _ from 'lodash';


import './index.css';

function Square(props) {
  return (
    <button className="square" onClick={props.onClick}>
      {props.value}
    </button>
  );
}

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(var i = 0; i < lines.length; i++) {
    const e = lines[i];
    if(squares[e[0]] && squares[e[0]] === squares[e[1]] && squares[e[1]] === squares[e[2]]) {
      return squares[e[0]];
    }
  };

  return null;
}

function calculateMovesLeft(squares) {
  return _.reject(squares, (s) => s).length
}

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 {
  handleClick(i) {
    const new_squares = this.state.history[0].slice();

    if(calculateWinner(new_squares)) {
      alert("There is already a winner!");
      return;
    } else if(new_squares[i]) {
      alert("There is already something in that square!");
      return;
    }
    new_squares[i] = this.state.currentPlayer;

    this.setState({
      history: [new_squares, ...this.state.history],
      currentPlayer: this.state.currentPlayer === 'X' ? 'O' : 'X',
    });
  }

  reset() {
    this.setState({
      history: [Array(9).fill(null)],
      currentPlayer: 'X',
    });
  }

  undo(n=1) {
    if(this.state.history.length < n + 1) {
      alert("Not enough state!");
      return;
    }

    this.setState({
      history: this.state.history.slice(n),
      currentPlayer: (this.state.history.length - n) % 2 ? 'X' : 'O',
    });
  }

  constructor(props) {
    super(props);

    this.state = {
      history: [Array(9).fill(null)],
      currentPlayer: 'X',
    };
  }

  render() {
    const winner = calculateWinner(this.state.history[0]);

    let status = 'Next player: ' + this.state.currentPlayer;
    if(winner) {
      status = "WINNER: " + winner;
    } else if(calculateMovesLeft(this.state.history[0]) === 0) {
      status = "DRAW!";
    }

    const moves = _.map(this.state.history.slice(1), (move, index) => {
      return (
        <li key={move}>
          <button onClick={() => this.undo(this.state.history.length - index - 1)}>Go back to move {index}</button>
        </li>
      )
    });

    return (
      <div>
        <div className="game">
          <div className="game-board">
            <Board
              squares={this.state.history[0]}
              currentPlayer={this.state.currentPlayer}
              onClick={(i) => this.handleClick(i)}
            />
          </div>
          <div className="game-info">
            <div>{ status }</div>
            <ul>{ moves }</ul>
          </div>
        </div>
        <button onClick={() => this.reset()}>Reset</button>
        <button onClick={() => this.undo()}>Undo</button>
      </div>
    );
  }
}

// ========================================

ReactDOM.render(
  <Game />,
  document.getElementById('root')
);