React in Practice: Understanding Hooks, State Management, and Next.js App Router

React isn't hard — but it's easy to misunderstand. This guide goes from real questions to real answers, explaining why things work the way they do, not just how to use them. What is React? React is a JavaScript library built by Meta for building user interfaces using a component model — splitting the UI into small, reusable pieces. function Greeting ({ name }) { return < h1 > Hello, { name } ! </ h1 >; } useState — Managing State useState stores data in the browser's RAM , in a region React manages internally. Each component has its own list of slots — React uses the order of hook calls to know which slot belongs to which useState . const [ count , setCount ] = useState ( 0 ); Why use the callback form prev => prev + 1 ? Because setState doesn't update immediately — React processes updates in batches. If you call it multiple times in a row: // ❌ Called 3 times → only increments by 1, because all calls use the same stale snapshot setCount ( count + 1 ); setCount ( count + 1 ); setCount