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92 lines
2.2 KiB
Markdown
92 lines
2.2 KiB
Markdown
---
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title: useState Dispatch updaters for State That Depends on Current Value
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impact: MEDIUM
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impactDescription: avoids stale closures, prevents unnecessary re-renders
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tags: state, hooks, useState, callbacks
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---
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## Use Dispatch Updaters for State That Depends on Current Value
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When the next state depends on the current state, use a dispatch updater
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(`setState(prev => ...)`) instead of reading the state variable directly in a
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callback. This avoids stale closures and ensures you're comparing against the
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latest value.
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**Incorrect (reads state directly):**
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```tsx
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const [size, setSize] = useState<Size | undefined>(undefined)
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const onLayout = (e: LayoutChangeEvent) => {
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const { width, height } = e.nativeEvent.layout
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// size may be stale in this closure
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if (size?.width !== width || size?.height !== height) {
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setSize({ width, height })
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}
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}
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```
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**Correct (dispatch updater):**
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```tsx
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const [size, setSize] = useState<Size | undefined>(undefined)
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const onLayout = (e: LayoutChangeEvent) => {
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const { width, height } = e.nativeEvent.layout
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setSize((prev) => {
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if (prev?.width === width && prev?.height === height) return prev
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return { width, height }
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})
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}
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```
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Returning the previous value from the updater skips the re-render.
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For primitive states, you don't need to compare values before firing a
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re-render.
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**Incorrect (unnecessary comparison for primitive state):**
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```tsx
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const [size, setSize] = useState<Size | undefined>(undefined)
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const onLayout = (e: LayoutChangeEvent) => {
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const { width, height } = e.nativeEvent.layout
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setSize((prev) => (prev === width ? prev : width))
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}
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```
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**Correct (sets primitive state directly):**
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```tsx
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const [size, setSize] = useState<Size | undefined>(undefined)
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const onLayout = (e: LayoutChangeEvent) => {
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const { width, height } = e.nativeEvent.layout
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setSize(width)
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}
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```
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However, if the next state depends on the current state, you should still use a
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dispatch updater.
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**Incorrect (reads state directly from the callback):**
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```tsx
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const [count, setCount] = useState(0)
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const onTap = () => {
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setCount(count + 1)
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}
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```
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**Correct (dispatch updater):**
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```tsx
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const [count, setCount] = useState(0)
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const onTap = () => {
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setCount((prev) => prev + 1)
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}
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```
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