React Updates UI BEFORE the API Responds?! 😳 | React 19 useOptimistic

How can React update the UI BEFORE the API response even finishes? 😳

In this React 19 tutorial, we build a real Follow/Unfollow feature using useOptimistic() and see exactly how optimistic UI works with the frontend, API, and backend data.

We first test what happens when you click Follow in a real application. Then we break down what is actually happening behind the scenes:

Click Follow → UI updates instantly ⚡ → API runs → Success = keep it / Failure = revert

In this video you'll understand:

⚡ What useOptimistic() actually does
🔄 Real State vs Optimistic State
👤 How Follow/Unfollow works in a full-stack application
🧠 useOptimistic() syntax and update function
📈 How follower count changes instantly
🔗 How updateOptimisticUser() receives its value
💾 How the API confirms the real state
↩️ What happens when the request fails
⚛️ How useState and useOptimistic work together

We are continuing our React 19 New Features series, so watching the previous videos on use(), Suspense, Actions and related React 19 concepts will help you get maximum value from this one.

The goal isn't just to memorize another React Hook — it's to understand how modern apps make actions such as Likes, Follow/Unfollow, comments, messages and cart updates feel instant.

Subscribe to Thapa Technical / Kodyfier for more React 19 tutorials. 🚀

#React19 #useOptimistic #ReactJS
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⌛TIMELINE⏳
00:00 Real-Life Experiment – Follow Button
01:15 What Actually Happens Behind the Scenes?
03:30 What is useOptimistic() in React 19?
04:02 useOptimistic() Syntax Explained
04:50 How a Follow Button Works in Full Stack
05:15 Hooks We Need to Import
05:30 Must-Watch Videos Before useOptimistic()
07:05 Creating the Real User State with useState()
09:30 Creating Optimistic Data for Instant UI
13:10 What Value Do We Pass When Follow is Clicked?
15:20 Understanding the Optimistic Update Function
17:20 Follow & Unfollow Updating Instantly
20:20 Final JSX Code
21:15 useOptimistic() – Final Recap