Setting the file. One moment.
Skill 04 · Vercel React Native Skills
Subchapter 4.26
rules/react-state-dispatcher.mdMarkdown2 KBView on GitHub
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)
}