Update OAuth2-Backend-Approach.md

OAuth2-Backend-Approach.md

@@ -322,3 +322,64 @@ app.get('/api/dashboard-data', (req, res) => {
   }
 });
 ```
+
+
+# Refresh token
+
+```js
+app.get('/api/dashboard-data', async (req, res) => {
+  const authToken = req.cookies.auth_token;
+  const refreshToken = await getRefreshTokenFromDB(); // Get from your DB
+
+  try {
+    //
+    // If the token is still valid
+    //
+    const userData = verifyToken(authToken);
+    const dashboardData = await getDashboardData(userData);
+    res.json(dashboardData);
+  } catch (err) {
+    if (err.name === 'TokenExpiredError') {
+      try {
+        //
+        // Get new tokens using refresh token
+        //
+        const response = await fetch('https://oauth2.googleapis.com/token', {
+          method: 'POST',
+          body: JSON.stringify({
+            refresh_token: refreshToken,
+            client_id: CLIENT_ID,
+            client_secret: CLIENT_SECRET,
+            grant_type: 'refresh_token'
+          })
+        });
+
+        const { access_token } = await response.json();
+
+        //
+        // Set new cookie with new access token
+        //
+        res.cookie('auth_token', access_token, {
+          httpOnly: true,
+          secure: true,
+          sameSite: 'strict'
+        });
+
+        //
+        // Continue with the original request using new token
+        //
+        const userData = verifyToken(access_token);
+        const dashboardData = await getDashboardData(userData);
+        res.json(dashboardData);
+      } catch (refreshError) {
+        //
+        // If refresh fails, user needs to login again
+        //
+        res.status(401).json({ error: 'Session expired' });
+      }
+    } else {
+      res.status(401).json({ error: 'Invalid token' });
+    }
+  }
+});
+```