Authentication sounds simple… until you actually have to implement it. At first, it feels easy: user logs in, backend gives a token, frontend stores it somewhere, and boom — done. Then reality enters the chat: Where should I store the token? LocalStorage or cookies? Why is my cookie not being set? Why does it work in Postman but not in the browser? Why am I debugging CORS at 2 AM? If you’ve been there, welcome. This blog will explain client-side authentication vs server-side authentication , how LocalStorage auth works, how cookie-based auth works, and how to properly set it up in both backend and frontend. No unnecessary theory. Just practical understanding. Table of Contents The Basic Authentication Flow Client-Side Auth Using LocalStorage Server-Side Auth Using Cookies Backend Setup for Cookie Authentication Frontend Access with Cookies LocalStorage vs Cookies: Which One is Better? Common Mistakes Developers Make Final Thoughts 1.…