determine a winner

src/index.js

@@ -22,7 +22,12 @@ function Square(props){
     handleClick(i) {
         const squares = this.state.squares.slice();
         squares[i] = this.state.xIsNext ? 'X' : 'O';
-        this.setState({squares: squares,xIsNext: !this.state.xIsNext});
+        this.setState(
+          {
+              squares: squares,
+              xIsNext: !this.state.xIsNext
+          }
+        );
     }
 
     renderSquare(i) {
@@ -39,7 +44,13 @@ function Square(props){
     }
   
     render() {
-      const status = 'Next player: ' + (this.state.xIsNext ? 'X' : 'O');
+      const winner = calculateWinner(this.state.squares);
+      let status;
+      if (winner) {
+        status = 'Winner: ' + winner;
+      } else {
+        status = 'Next Player: ' + (this.state.xIsNext ? 'X' : 'O')
+      }
   
       return (
         <div>
@@ -87,3 +98,22 @@ function Square(props){
     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;
+  }