Basic tic-tac-toe game

src/index.css

@@ -1,14 +1,50 @@
 body {
-  margin: 0;
+  font: 14px "Century Gothic", Futura, sans-serif;
+  margin: 20px;
+}
+
+ol, ul {
+  padding-left: 30px;
+}
+
+.board-row:after {
+  clear: both;
+  content: "";
+  display: table;
+}
+
+.status {
+  margin-bottom: 10px;
+}
+
+.square {
+  background: #fff;
+  border: 1px solid #999;
+  float: left;
+  font-size: 24px;
+  font-weight: bold;
+  line-height: 34px;
+  height: 34px;
+  margin-right: -1px;
+  margin-top: -1px;
   padding: 0;
-  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", "Roboto", "Oxygen",
-    "Ubuntu", "Cantarell", "Fira Sans", "Droid Sans", "Helvetica Neue",
-    sans-serif;
-  -webkit-font-smoothing: antialiased;
-  -moz-osx-font-smoothing: grayscale;
+  text-align: center;
+  width: 34px;
+}
+
+.square:focus {
+  outline: none;
+}
+
+.kbd-navigation .square:focus {
+  background: #ddd;
+}
+
+.game {
+  display: flex;
+  flex-direction: row;
 }
 
-code {
-  font-family: source-code-pro, Menlo, Monaco, Consolas, "Courier New",
-    monospace;
+.game-info {
+  margin-left: 20px;
 }

src/index.js

@@ -1,12 +1,146 @@
 import React from 'react';
 import ReactDOM from 'react-dom';
 import './index.css';
-import App from './App';
-import * as serviceWorker from './serviceWorker';
 
-ReactDOM.render(<App />, document.getElementById('root'));
+function Square(props) {
+  return (
+    <button className="square" onClick={props.onClick}>
+      {props.value}
+    </button>
+  )
+}
 
-// If you want your app to work offline and load faster, you can change
-// unregister() to register() below. Note this comes with some pitfalls.
-// Learn more about service workers: http://bit.ly/CRA-PWA
-serviceWorker.unregister();
+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;
+    } 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;
+}