useReducerAtom

useReducerAtom is a custom hook to apply a reducer to a primitive atom.

It’s useful to change the update behavior temporarily. Also, consider atomWithReducer for the atom-level solution.

import { useCallback } from 'react'
import { useAtom } from 'jotai'
import type { PrimitiveAtom } from 'jotai'

export function useReducerAtom<Value, Action>(
  anAtom: PrimitiveAtom<Value>,
  reducer: (v: Value, a: Action) => Value,
) {
  const [state, setState] = useAtom(anAtom)
  const dispatch = useCallback(
    (action: Action) => setState((prev) => reducer(prev, action)),
    [setState, reducer],
  )
  return [state, dispatch] as const
}

Example Usage

import { atom } from 'jotai'

const countReducer = (prev, action) => {
  if (action.type === 'inc') return prev + 1
  if (action.type === 'dec') return prev - 1
  throw new Error('unknown action type')
}

const countAtom = atom(0)

const Counter = () => {
  const [count, dispatch] = useReducerAtom(countAtom, countReducer)
  return (
    <div>
      {count}
      <button onClick={() => dispatch({ type: 'inc' })}>+1</button>
      <button onClick={() => dispatch({ type: 'dec' })}>-1</button>
    </div>
  )
}

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