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useShallow ⚛️

How to memoize selector functions

useShallow is a React Hook that lets you optimize re-renders.

const memoizedSelector = useShallow(selector)

Signature

 
useShallow<T, U = T>(selectorFn: (state: T) => U): (state: T) => U

Reference

useShallow(selectorFn)

Parameters
  • selectorFn: A function that lets you return data that is based on current state.

Returns

useShallow returns a memoized version of a selector function using a shallow comparison for memoization.

Usage

Writing a memoized selector

First, we need to setup a store to hold the state for the bear family. In this store, we define three properties: papaBear, mamaBear, and babyBear, each representing a different member of the bear family and their respective oatmeal pot sizes.

import { create } from 'zustand'

type BearFamilyMealsStore = {
  [key: string]: string
}

const useBearFamilyMealsStore = create<BearFamilyMealsStore>()(() => ({
  papaBear: 'large porridge-pot',
  mamaBear: 'middle-size porridge pot',
  babyBear: 'A little, small, wee pot',
}))

Next, we'll create a BearNames component that retrieves the keys of our state (the bear family members) and displays them.

function BearNames() {
  const names = useBearFamilyMealsStore((state) => Object.keys(state))

  return <div>{names.join(', ')}</div>
}

Next, we will create a UpdateBabyBearMeal component that periodically updates baby bear's meal choice.

const meals = [
  'A tiny, little, wee bowl',
  'A small, petite, tiny pot',
  'A wee, itty-bitty, small bowl',
  'A little, petite, tiny dish',
  'A tiny, small, wee vessel',
  'A small, little, wee cauldron',
  'A little, tiny, small cup',
  'A wee, small, little jar',
  'A tiny, wee, small pan',
  'A small, wee, little crock',
]

function UpdateBabyBearMeal() {
  useEffect(() => {
    const timer = setInterval(() => {
      useBearFamilyMealsStore.setState({
        tinyBear: meals[Math.floor(Math.random() * (meals.length - 1))],
      })
    }, 1000)

    return () => {
      clearInterval(timer)
    }
  }, [])

  return null
}

Finally, we combine both components in the App component to see them in action.

export default function App() {
  return (
    <>
      <UpdateTinyBearPorridge />
      <BearNames />
    </>
  )
}

Here is what the code should look like:

import { useEffect } from 'react'
import { create } from 'zustand'

type BearFamilyMealsStore = {
  [key: string]: string
}

const useBearFamilyMealsStore = create<BearFamilyMealsStore>()(() => ({
  papaBear: 'large porridge-pot',
  mamaBear: 'middle-size porridge pot',
  babyBear: 'A little, small, wee pot',
}))

const meals = [
  'A tiny, little, wee bowl',
  'A small, petite, tiny pot',
  'A wee, itty-bitty, small bowl',
  'A little, petite, tiny dish',
  'A tiny, small, wee vessel',
  'A small, little, wee cauldron',
  'A little, tiny, small cup',
  'A wee, small, little jar',
  'A tiny, wee, small pan',
  'A small, wee, little crock',
]

function UpdateBabyBearMeal() {
  useEffect(() => {
    const timer = setInterval(() => {
      useBearFamilyMealsStore.setState({
        tinyBear: meals[Math.floor(Math.random() * (meals.length - 1))],
      })
    }, 1000)

    return () => {
      clearInterval(timer)
    }
  }, [])

  return null
}

function BearNames() {
  const names = useBearFamilyMealsStore((state) => Object.keys(state))

  return <div>{names.join(', ')}</div>
}

export default function App() {
  return (
    <>
      <UpdateBabyBearMeal />
      <BearNames />
    </>
  )
}

Everything might look fine, but there’s a small problem: the BearNames component keeps re-rendering even if the names haven’t changed. This happens because the component re-renders whenever any part of the state changes, even if the specific part we care about (the list of names) hasn’t changed.

To fix this, we use useShallow to make sure the component only re-renders when the actual keys of the state change:

function BearNames() {
  const names = useBearFamilyStore(useShallow((state) => Object.keys(state)))

  return <div>{names.join(', ')}</div>
}

Here is what the code should look like:

import { useEffect } from 'react'
import { create } from 'zustand'
import { useShallow } from 'zustand/react/shallow'

type BearFamilyMealsStore = {
  [key: string]: string
}

const useBearFamilyMealsStore = create<BearFamilyMealsStore>()(() => ({
  papaBear: 'large porridge-pot',
  mamaBear: 'middle-size porridge pot',
  babyBear: 'A little, small, wee pot',
}))

const meals = [
  'A tiny, little, wee bowl',
  'A small, petite, tiny pot',
  'A wee, itty-bitty, small bowl',
  'A little, petite, tiny dish',
  'A tiny, small, wee vessel',
  'A small, little, wee cauldron',
  'A little, tiny, small cup',
  'A wee, small, little jar',
  'A tiny, wee, small pan',
  'A small, wee, little crock',
]

function UpdateBabyBearMeal() {
  useEffect(() => {
    const timer = setInterval(() => {
      useBearFamilyMealsStore.setState({
        tinyBear: meals[Math.floor(Math.random() * (meals.length - 1))],
      })
    }, 1000)

    return () => {
      clearInterval(timer)
    }
  }, [])

  return null
}

function BearNames() {
  const names = useBearFamilyMealsStore(
    useShallow((state) => Object.keys(state)),
  )

  return <div>{names.join(', ')}</div>
}

export default function App() {
  return (
    <>
      <UpdateBabyBearMeal />
      <BearNames />
    </>
  )
}

By using useShallow, we optimized the rendering process, ensuring that the component only re-renders when necessary, which improves overall performance.

Troubleshooting

TBD

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