Add: Time-Travel Function.

src/App.tsx

@@ -5,15 +5,38 @@ interface SquareProps {
   onSquareClick: () => void;
 }
 
+interface BoardProps {
+  XIsNext: boolean;
+  squares: Array<SquareProps["value"]>;
+  onPlay: (nextSquares: Array<SquareProps["value"]>) => void;
+}
+
 function Square({
   value, onSquareClick}: SquareProps) {
   return <button type="button" onClick={onSquareClick} className="square">{value}</button>
 }
 
-export default function Board() { 
-  const [XIsNext, setXIsNext] = useState(true);
-  const [squares, setSquares] = useState(Array(9).fill(null));
+function calculateWinner(squares: BoardProps["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;
+}
 
+export function Board({XIsNext, squares, onPlay}: BoardProps) { 
   const winner = calculateWinner(squares);
   let status;
   if (winner) {
@@ -32,8 +55,7 @@ export default function Board() {
     } else {
       nextSquares[i] = "O";
     }
-    setSquares(nextSquares);
-    setXIsNext(!XIsNext);
+    onPlay(nextSquares)
   }
 
   return(
@@ -58,22 +80,48 @@ export default function Board() {
   )
 }
 
-function calculateWinner(squares: Array<SquareProps["value"]>) {
-  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];
+export default function Game() {
+  const [history, setHistory] = useState([Array(9).fill(null)]);
+  const [currentMove, setCurrentMove] = useState(0);
+  
+  const XIsNext = currentMove % 2 === 0;
+  const currentSquares = history[currentMove];
+  
+  function handlePlay(nextSquares: Array<SquareProps["value"]>) {
+    const nextHistory = [...history.slice(0, currentMove + 1), nextSquares];
+    
+    setHistory(nextHistory);
+    setCurrentMove(nextHistory.length - 1);
   }
+
+  function jumpTo(nextMove: number) {
+    setCurrentMove(nextMove);
   }
-  return null;
+  
+  const moves = history.map((_squares, move) => {
+    let description;
+    if (move > 0) {
+      description = 'Go to move #' + move;
+    } else {
+      description = 'Go to game start';
+    }
+    return (
+      <li key={move}>
+        <button onClick={() => jumpTo(move)}>{description}</button>
+      </li>
+    );
+  })
+
+  return (
+    <div className="game">
+      <div className="game-board">
+        <Board XIsNext={XIsNext} squares={currentSquares} onPlay={handlePlay}/>
+      </div>
+      <div className="game-info">
+        <ol>
+          {moves}
+        </ol>
+      </div>
+    </div>
+  )
 }

src/index.tsx

@@ -3,7 +3,7 @@ import {createRoot,} from 'react-dom/client';
 import App from './App';
 import './App.css'
 
-const root = createRoot(document.getElementById("root"))
+const root = createRoot(document.getElementById("root") as HTMLElement)
 
 root.render(
     <StrictMode>