React Fundamentals Explained: Virtual DOM, Reconciliation, and Fiber

React was created to solve the performance problems of directly manipulating the browser's DOM, which triggers costly layout recalculations with every change. To address this, React introduced the Virtual DOM — a lightweight in-memory copy of the UI that allows changes to be computed cheaply before minimal updates are applied to the real DOM. This process of comparing old and new Virtual DOM trees is called reconciliation, with the difference-finding step known as diffing. Before React's Fiber engine, rendering was synchronous and blocking, meaning large updates could freeze the UI and delay responses to user input. Fiber replaced this model by making rendering interruptible, allowing React to prioritize urgent updates like keystrokes over heavier background renders.