What is useEffect and its cleanup?

useEffect handles side effects in functional components. It runs after render and can optionally clean up.\n\nBasic usage:\nuseEffect(() => {\n // Side effect\n document.title = "New Title";\n}, [dependency]); // Dependency array\n\nWith cleanup:\nuseEffect(() => {\n const timer = setInterval(() => {\n console.log("Timer tick");\n }, 1000);\n \n // Cleanup function\n return () => {\n clearInterval(timer);\n };\n}, []);\n\nDependency array:\n- [] = run once (componentDidMount)\n- [value] = run when value changes\n- no array = run on every render