Understanding React useEffect: Synchronization, Dependencies, and Cleanup Explained

React's useEffect hook is designed to synchronize a component with external systems such as network connections, timers, WebSockets, and third-party libraries. It runs after React commits changes to the DOM, and its behavior is controlled by a dependency array that determines when re-synchronization is needed. React compares dependency values using Object.is, and any reactive value read inside the effect should be listed as a dependency to avoid subtle bugs. When a dependency changes, React first runs the previous effect's cleanup function before executing the new setup, ensuring external systems stay in sync. Misunderstanding these mechanics — such as omitting dependencies or suppressing lint warnings — can lead to stale data, infinite loops, or unexpected re-renders.