React Fiber, Concurrency, and Reconciliation

The source provides an extensive overview of React's internal architecture, focusing on the Fiber reconciliation engine and the concept of concurrent rendering. It explains how declarative JSX is compiled into lightweight React element objects that form the Virtual DOM, which is then mapped to Fiber objects that manage work and state. The text details the two primary phases of updates—the render phase (where changes are calculated via diffing with keys and types) and the synchronous commit phase (where changes are applied to the real DOM). Finally, it clarifies React's modern concurrency model using a lane-based scheduler to prioritize updates (like urgent user inputs over background transitions) and discusses the internal implementation of major APIs, including Hooks and Suspense.