When someone wins, highlight the three squares that caused the win

src/index.css

@@ -36,6 +36,10 @@ ol, ul {
     outline: none;
 }
 
+.square.in-win-line {
+    background-color: #fff300;
+}
+
 .kbd-navigation .square:focus {
     background: #ddd;
 }

src/index.js

@@ -3,13 +3,17 @@ import ReactDOM from 'react-dom';
 import './index.css';
 
 /*TODO
-    When someone wins, highlight the three squares that caused the win.
+
     When no one wins, display a message about the result being a draw.
 */
 
 function Square(props) {
+    const classNames = ['square'];
+    if (props.isInWinLine) {
+        classNames.push('in-win-line');
+    }
     return (
-        <button className="square" onClick={props.onClick}>
+        <button className={classNames.join(' ')} onClick={props.onClick}>
             {props.value}
         </button>
     );
@@ -26,6 +30,7 @@ function Board(props) {
                     key={index}
                     value={props.squares[index]}
                     onClick={() => props.handleClick(index)}
+                    isInWinLine={props.winLine?.indexOf(index) >= 0}
                 />
             );
         }
@@ -55,7 +60,7 @@ class Game extends React.Component {
     playTurn(i) {
         const history = this.state.history.slice(0, this.state.currentMoveIndex + 1);
         const currentState = history[history.length - 1];
-        if (currentState.squares[i] || calculateWinner(currentState.squares)) {
+        if (currentState.squares[i] || calculateWinState(currentState.squares)) {
             return;
         }
 
@@ -91,9 +96,9 @@ class Game extends React.Component {
         const history = this.state.history;
         const currentState = history[this.state.currentMoveIndex];
         let status;
-        const winner = calculateWinner(currentState.squares);
-        if (winner) {
-            status = `Winner: ${winner}`;
+        const winState = calculateWinState(currentState.squares);
+        if (winState) {
+            status = `Winner: ${winState.winner}`;
         } else {
             const nextPlayer = this.state.xIsNext ? 'X' : 'O';
             status = `Next player ${nextPlayer}`;
@@ -121,6 +126,7 @@ class Game extends React.Component {
                         squares={currentState.squares}
                         handleClick={(i) => this.playTurn(i)}
                         xIsNext={this.state.xIsNext}
+                        winLine={winState?.line}
                     />
                 </div>
                 <div className="game-info">
@@ -133,7 +139,7 @@ class Game extends React.Component {
     }
 }
 
-function calculateWinner(squares) {
+function calculateWinState(squares) {
     const lines = [
         [0, 1, 2],
         [3, 4, 5],
@@ -147,7 +153,7 @@ function calculateWinner(squares) {
     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 {winner: squares[a], line: lines[i]};
         }
     }
     return null;