1---
2title: useState Dispatch updaters for State That Depends on Current Value
3impact: MEDIUM
4impactDescription: avoids stale closures, prevents unnecessary re-renders
5tags: state, hooks, useState, callbacks
6---
7 
8## Use Dispatch Updaters for State That Depends on Current Value
9 
10When the next state depends on the current state, use a dispatch updater
11(`setState(prev => ...)`) instead of reading the state variable directly in a
12callback. This avoids stale closures and ensures you're comparing against the
13latest value.
14 
15**Incorrect (reads state directly):**
16 
17```tsx
18const [size, setSize] = useState<Size | undefined>(undefined)
19 
20const onLayout = (e: LayoutChangeEvent) => {
21  const { width, height } = e.nativeEvent.layout
22  // size may be stale in this closure
23  if (size?.width !== width || size?.height !== height) {
24    setSize({ width, height })
25  }
26}
27```
28 
29**Correct (dispatch updater):**
30 
31```tsx
32const [size, setSize] = useState<Size | undefined>(undefined)
33 
34const onLayout = (e: LayoutChangeEvent) => {
35  const { width, height } = e.nativeEvent.layout
36  setSize((prev) => {
37    if (prev?.width === width && prev?.height === height) return prev
38    return { width, height }
39  })
40}
41```
42 
43Returning the previous value from the updater skips the re-render.
44 
45For primitive states, you don't need to compare values before firing a
46re-render.
47 
48**Incorrect (unnecessary comparison for primitive state):**
49 
50```tsx
51const [size, setSize] = useState<Size | undefined>(undefined)
52 
53const onLayout = (e: LayoutChangeEvent) => {
54  const { width, height } = e.nativeEvent.layout
55  setSize((prev) => (prev === width ? prev : width))
56}
57```
58 
59**Correct (sets primitive state directly):**
60 
61```tsx
62const [size, setSize] = useState<Size | undefined>(undefined)
63 
64const onLayout = (e: LayoutChangeEvent) => {
65  const { width, height } = e.nativeEvent.layout
66  setSize(width)
67}
68```
69 
70However, if the next state depends on the current state, you should still use a
71dispatch updater.
72 
73**Incorrect (reads state directly from the callback):**
74 
75```tsx
76const [count, setCount] = useState(0)
77 
78const onTap = () => {
79  setCount(count + 1)
80}
81```
82 
83**Correct (dispatch updater):**
84 
85```tsx
86const [count, setCount] = useState(0)
87 
88const onTap = () => {
89  setCount((prev) => prev + 1)
90}
91```
92