When do you NOT need `useEffect`?

Summary

You don't need effects to derive data from props/state (compute during render), to handle user events (do it in event handlers), to reset state on prop change (use key), to chain state updates, or to notify parents (call callbacks in handlers).

Longer Answer

Effects are for synchronizing with external systems (subscriptions, network, DOM APIs). Unnecessary effects cause extra renders and bugs.

Key Points

• Derive during render
• Use event handlers for events
• key to reset state
• Effects only for external sync