useEffectOnce

Requirements

• Implement useEffectOnce(effect), returning undefined. effect returns either undefined or a cleanup function.
• In normal mounting, run effect after the component commits. Ordinary rerenders, including receiving a new effect callback, must not run it again.
• If the initial effect returns cleanup, React must call that cleanup on unmount. No cleanup is required for an undefined return.
• A fresh mount runs its own effect. Instances are independent.
• Once refers to a mount-only effect with an empty dependency array, not suppression of React Strict Mode. In development Strict Mode, preserve React's setup → cleanup → setup replay and final unmount cleanup. Do not use a ref guard to bypass replay. Effects do not run during server rendering.

Examples

Input: useEffectOnce(() => { console.log("mounted"); return () => console.log("unmounted"); });
Output: Runs setup on mount and cleanup on unmount; rerenders do not repeat setup.