In this React useEffect video, I break down what “side effects” really mean in React function components and why useEffect exists in the first place. React is component-based, and useEffect is the hook that lets me run things after render—like data fetching, subscriptions, or manually changing the DOM. The key idea is simple: I pass a function to useEffect, React remembers it, and then calls it after it finishes updating the DOM. I also explain how useEffect behaves by default: it runs after every render, including the very first one. That’s different from class components where some effects might feel like they run only once. To control when an effect runs, I use the dependency array (the second argument). If dependencies change, the effect runs again; if the array is empty, it runs once—similar to componentDidMount. Finally, I cover tips and common mistakes I see all the time: forgetting cleanup (unsubscribe in a return function to avoid leaks), putting inline functions into the dependency array causing unnecessary renders (use useCallback or useRef), and doing heavy computations inside useEffect (use useMemo instead). Used carefully, useEffect makes code more readable, maintainable, and performant.