React 19 internals

When I first got introduced to React, it felt like magic. I was just describing what I wanted, and somehow React handled all the heavy lifting. Take a simple example — showing text when a button is clicked. In React, that’s it: import { useState } from ' react ' function App () { const [ visible , setVisible ] = useState ( false ); return ( < div > < button onClick = { () => setVisible ( true ) } > Click to see </ button > { visible && < p > Hi, Its Visible </ p > } </ div > ); }; export default App ; No getElementById. No addEventListener . No manually toggling .style.display . In vanilla JS you're telling the browser how to do something step by step. In React you just declare what the UI should look like for a given state, and React figures out the rest. That's the heart of declarative programming. I had so many questions. How is this even possible? How does it know what to update and when? So I dug into the React source code — and here’s the thing: it’s not magic at all. It’s a bril