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.…