useOnceEffect

A Hook that avoids React18 useEffect run twice

useOnceEffect is a variant of useEffect that guarantees the effect callback executes only once, even under React 18’s Strict Mode which intentionally double-invokes effects during development. It uses a WeakSet-based tracking mechanism to detect and skip duplicate invocations. The API is identical to useEffect — it accepts an effect callback and an optional dependency array.

When to Use

• Performing side effects that must run exactly once (e.g., sending an analytics event, initializing a third-party library) and cannot tolerate React 18 Strict Mode double-firing
• Protecting against duplicate API calls on mount in development mode with <StrictMode> enabled
• Replacing manual useRef-based “already ran” guards with a cleaner API

Notes

• React 18 Strict Mode: In development, React 18 mounts, unmounts, and re-mounts components to surface impure effects. useOnceEffect prevents the second invocation by tracking the effect reference in a WeakSet.
• Production behavior: In production builds where Strict Mode double-invocation does not occur, useOnceEffect behaves identically to useEffect.
• See also useOnceLayoutEffect for the same behavior using useLayoutEffect timing, and useMount for a simpler mount-only callback.

Usage

function Demo() {
  const [effect, setEffect] = useState(0);
  const [onceEffect, setOnceEffect] = useState(0);

  useOnceEffect(() => {
    setOnceEffect(onceEffect => onceEffect + 1);
  }, []);

  useEffect(() => {
    setEffect(effect => effect + 1);
  }, []);

  return (
    <div>
      <div>onceEffect: {onceEffect}</div>
      <br />
      <div>effect: {effect}</div>
    </div>
  );
};