Effects

useEffect lets you run code after a component renders. Use it for things that need to reach outside of React: updating the document title, setting up a timer, subscribing to an event, or starting a network request.

useEffect takes two arguments: the effect function, and a dependency array. The dependency array controls when the effect re-runs. If you pass [count], the effect runs whenever count changes.

An empty array [] means the effect runs once — after the first render only. No array means it runs after every render. Missing the dependency array when you meant to use [] is a very common bug.

Effects can return a cleanup function. React calls it before running the effect again, and when the component unmounts. Use cleanup to clear timers, cancel subscriptions, and avoid memory leaks.