Update OAuth2-Front-Approach.md

OAuth2-Front-Approach.md

@@ -363,11 +363,89 @@ const fetchUserData = async (accessToken) => {
 <br><br><br>
 
 
+# 5. [Frontend] Refresh the Token
+
+If the access token is expired, the frontend will receive an error response from Google when attempting to fetch user data
+
+<details>
+<summary><h3>Frontend HTTP GET Request to Google</h3></summary>
+
+```bash
+GET https://www.googleapis.com/oauth2/v3/userinfo
+Authorization: Bearer ACCESSTOKEN6zXZkHi2XITkDoOVACCESSTOKEN
+```
+
+</details>
+
+<details>
+<summary><h3>Google HTTP Response to Frontend</h3></summary>
+
+```json
+{
+  "error": "invalid_token",
+  "error_description": "The access token expired"
+}
+```
+
+</details>
 
 
+<details>
+<summary><h3>Frontend HTTP POST Refresh token to Backend</h3></summary>
+
+```bash
+
+```
+
+</details>
+
+<details>
+<summary><h3>Frontend Code</h3></summary>
+
+```js
+const refreshAccessToken = async (refreshToken) => {
+  try {
+    const response = await fetch('/api/refresh-token', {
+      method: 'POST',
+      headers: {
+        'Content-Type': 'application/json',
+      },
+      body: JSON.stringify({ refresh_token: refreshToken }),
+    });
+
+    if (!response.ok) {
+      throw new Error('Failed to refresh access token');
+    }
+
+    const data = await response.json();
+    return data.access_token;  // New access token
+  } catch (error) {
+    console.error('Error refreshing access token:', error);
+  }
+};
+```
+
+</details>
 
 
+<details>
+<summary><h3>Frontend Code</h3></summary>
 
+```js
+const response = await fetch('https://oauth2.googleapis.com/token', {
+  method: 'POST',
+  headers: {
+    'Content-Type': 'application/x-www-form-urlencoded',
+  },
+  body: new URLSearchParams({
+    client_id: 'YOUR_GOOGLE_CLIENT_ID',
+    client_secret: 'YOUR_GOOGLE_CLIENT_SECRET',
+    refresh_token: refreshToken,
+    grant_type: 'refresh_token',
+  }),
+});
 
-
-
+const data = await response.json();
+const newAccessToken = data.access_token;  // New access token
+```
+</details>