atomWithCompare

atomWithCompare creates atom that triggers updates when custom compare function areEqual(prev, next) is false.

This can help you avoid unwanted re-renders by ignoring state changes that don't matter to your application.

Note: Jotai uses Object.is internally to compare values when changes occur. If areEqual(a, b) returns false, but Object.is(a, b) returns true, Jotai will not trigger an update.

import { atomWithReducer } from 'jotai/utils'

export function atomWithCompare<Value>(
  initialValue: Value,
  areEqual: (prev: Value, next: Value) => boolean
) {
  return atomWithReducer(initialValue, (prev: Value, next: Value) => {
    if (areEqual(prev, next)) {
      return prev
    }

    return next
  })
}

Here's how you'd use it to make an atom that ignores updates that are shallow-equal:

import { atomWithCompare } from 'XXX'
import { shallowEquals } from 'YYY'
import { CSSProperties } from 'react'

const styleAtom = atomWithCompare<CSSProperties>(
  { backgroundColor: 'blue' },
  shallowEquals
)

In a component:

const StylePreview = () => {
  const [styles, setStyles] = useAtom(styleAtom)

  return (
    <div>
      <div styles={styles}>Style preview</div>

      {/* Clicking this button twice will only trigger one render */}
      <button onClick={() => setStyles({ ...styles, backgroundColor: 'red' })}>
        Set background to red
      </button>

      {/* Clicking this button twice will only trigger one render */}
      <button onClick={() => setStyles({ ...styles, fontSize: 32 })}>
        Enlarge font
      </button>
    </div>
  )
}