HausApp/.agents/skills/vercel-react-native-skills/rules/react-state-dispatcher.md

title, impact, impactDescription, tags

title	impact	impactDescription	tags
useState Dispatch updaters for State That Depends on Current Value	MEDIUM	avoids stale closures, prevents unnecessary re-renders	state, hooks, useState, callbacks

Use Dispatch Updaters for State That Depends on Current Value

When the next state depends on the current state, use a dispatch updater (setState(prev => ...)) instead of reading the state variable directly in a callback. This avoids stale closures and ensures you're comparing against the latest value.

Incorrect (reads state directly):

const [size, setSize] = useState<Size | undefined>(undefined);

const onLayout = (e: LayoutChangeEvent) => {
  const { width, height } = e.nativeEvent.layout;
  // size may be stale in this closure
  if (size?.width !== width || size?.height !== height) {
    setSize({ width, height });
  }
};

Correct (dispatch updater):

const [size, setSize] = useState<Size | undefined>(undefined);

const onLayout = (e: LayoutChangeEvent) => {
  const { width, height } = e.nativeEvent.layout;
  setSize((prev) => {
    if (prev?.width === width && prev?.height === height) return prev;
    return { width, height };
  });
};

Returning the previous value from the updater skips the re-render.

For primitive states, you don't need to compare values before firing a re-render.

Incorrect (unnecessary comparison for primitive state):

const [size, setSize] = useState<Size | undefined>(undefined);

const onLayout = (e: LayoutChangeEvent) => {
  const { width, height } = e.nativeEvent.layout;
  setSize((prev) => (prev === width ? prev : width));
};

Correct (sets primitive state directly):

const [size, setSize] = useState<Size | undefined>(undefined);

const onLayout = (e: LayoutChangeEvent) => {
  const { width, height } = e.nativeEvent.layout;
  setSize(width);
};

However, if the next state depends on the current state, you should still use a dispatch updater.

Incorrect (reads state directly from the callback):

const [count, setCount] = useState(0);

const onTap = () => {
  setCount(count + 1);
};

Correct (dispatch updater):

const [count, setCount] = useState(0);

const onTap = () => {
  setCount((prev) => prev + 1);
};