prevent-rerenders-with-use-shallow.md

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Prevent rerenders with useShallow	15

When you need to subscribe to a computed state from a store, the recommended way is to use a selector.

The computed selector will cause a rerender if the output has changed according to Object.is.

In this case you might want to use useShallow to avoid a rerender if the computed value is always shallow equal the previous one.

Example

We have a store that associates to each bear a meal and we want to render their names.

import { create } from 'zustand'

const useMeals = create(() => ({
  papaBear: 'large porridge-pot',
  mamaBear: 'middle-size porridge pot',
  littleBear: 'A little, small, wee pot',
}))

export const BearNames = () => {
  const names = useMeals((state) => Object.keys(state))

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

Now papa bear wants a pizza instead:

useMeals.setState({
  papaBear: 'a large pizza',
})

This change causes BearNames rerenders even though the actual output of names has not changed according to shallow equal.

We can fix that using useShallow!

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

const useMeals = create(() => ({
  papaBear: 'large porridge-pot',
  mamaBear: 'middle-size porridge pot',
  littleBear: 'A little, small, wee pot',
}))

export const BearNames = () => {
  const names = useMeals(useShallow((state) => Object.keys(state)))

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

Now they can all order other meals without causing unnecessary rerenders of our BearNames component.