A [React](/courses/reactjs) login flow usually collects credentials, sends them to the backend, handles loading and errors, updates authentication state after success, and redirects or updates the interface for the authenticated user. ### Basic Login Flow ```text User enters credentials ↓ Submit login form ↓ Show loading state ↓ POST credentials to backend / \ success failure ↓ ↓ update show error session ↓ load user ↓ show authenticated UI ``` ### Login Form Example ```jsx function LoginForm() { const [email, setEmail] = useState(""); const [password, setPassword] = useState(""); const [loading, setLoading] = useState(false); const [error, setError] = useState(null); const { refreshUser } = useAuth(); async function handleSubmit(event) { event.preventDefault(); try { setLoading(true); setError(null); const response = await fetch("/api/auth/login", { method: "POST", credentials: "include", headers: { "Content-Type": "application/json" }, body: JSON.stringify({ email, password }) }); if (!response.ok) { throw new Error("Invalid email or password"); } await refreshUser(); } catch (error) { setError(error.message); } finally { setLoading(false); } } return ( ); } ``` ### Why Refresh the Current User After Login? After successful authentication, the application usually needs safe user information such as: ```text id name email role permissions ``` A current-user endpoint might be: ```text GET /api/auth/me ``` The authentication provider can load it: ```jsx async function refreshUser() { const response = await fetch("/api/auth/me", { credentials: "include" }); if (!response.ok) { setUser(null); return; } const user = await response.json(); setUser(user); } ``` ### Check Authentication When the Application Starts A page refresh removes normal [React](/courses/reactjs) state. Therefore the application should not rely only on: ```jsx setUser(user); ``` from the original login request. On application startup, verify whether a valid session still exists: ```text Application loads ↓ GET /api/auth/me ↓ valid session? / \ yes no ↓ ↓ user logged out ``` ### Logout Flow With server-managed authentication, logout should usually tell the backend to invalidate or clear the session. ```jsx async function logout() { await fetch("/api/auth/logout", { method: "POST", credentials: "include" }); setUser(null); } ``` Do not only hide the user in React if the backend session still remains valid. ### Handle Expired Sessions A user can become unauthenticated while the application is open. For example: ```text Session expires ↓ API returns 401 ↓ clear authenticated state ↓ show sign-in UI ``` This behavior is often centralized in the authentication or API layer instead of implementing it separately in every page. ### Avoid Revealing Too Much in Login Errors Depending on the product's security requirements, a message such as: ```text Invalid email or password ``` may be preferable to separately revealing whether a particular account exists. ### Related Questions to Learn More ```text Forms and User Input → How Do You Handle Form Submission in React? → How Do You Handle Loading and Errors During React Form Submission? React API Integration → How Do You Make Authenticated API Requests in React? Error Handling → How Should You Handle Async Errors in React Event Handlers and Effects? React Accessibility → How Do You Build Accessible Forms in React? ``` ### Interview-Ready Answer A React login flow collects credentials, submits them securely to the backend, handles loading and failure states, and refreshes the authenticated-user state after success. The application should also verify the current session when it starts, handle session expiration, and call the backend during logout so the real session is invalidated rather than only clearing local UI state.