feat(util): atomWithDefault

docs/api/utils.md

@@ -180,6 +180,22 @@ const countReducerAtom = atomWithReducer(0, countReducer)
 
 https://codesandbox.io/s/react-typescript-forked-g3tsx
 
+## atomWithDefault
+
+Ref: https://github.com/pmndrs/jotai/issues/352
+
+This is a function to create a primitive atom.
+Its default value can be specified with a read function instead of a static initial value.
+
+```js
+import { atomWithDefault } from 'jotai/utils'
+
+const count1Atom = atom(1)
+const count2Atom = atomWithDefault((get) => get(count1Atom) * 2)
+```
+
+https://codesandbox.io/s/react-typescript-forked-unfro
+
 ## atomFamily
 
 Ref: https://github.com/pmndrs/jotai/issues/23

src/utils.ts

@@ -9,3 +9,4 @@ export { selectAtom } from './utils/selectAtom'
 export { useAtomCallback } from './utils/useAtomCallback'
 export { freezeAtom, atomFrozenInDev } from './utils/freezeAtom'
 export { splitAtom } from './utils/splitAtom'
+export { atomWithDefault } from './utils/atomWithDefault'

src/utils/atomWithDefault.ts

@@ -0,0 +1,26 @@
+import { atom, PrimitiveAtom } from 'jotai'
+import type { Read } from '../core/types'
+
+export function atomWithDefault<Value>(
+  getDefault: Read<Value>
+): PrimitiveAtom<Value> {
+  const overwrittenAtom = atom(false)
+  const anAtom: PrimitiveAtom<Value> = atom(
+    (get) => {
+      if (get(overwrittenAtom)) {
+        return get(anAtom)
+      }
+      return getDefault(get)
+    },
+    (get, set, update) => {
+      set(overwrittenAtom, true)
+      set(
+        anAtom,
+        typeof update === 'function'
+          ? (update as (prev: Value) => Value)(get(anAtom))
+          : update
+      )
+    }
+  )
+  return anAtom
+}

src/utils/atomWithReset.ts

@@ -6,7 +6,7 @@ export const RESET = Symbol()
 
 export function atomWithReset<Value>(initialValue: Value) {
   type Update = SetStateAction<Value> | typeof RESET
-  const anAtom: any = atom<Value, Update>(initialValue, (get, set, update) => {
+  const anAtom = atom<Value, Update>(initialValue, (get, set, update) => {
     if (update === RESET) {
       set(anAtom, initialValue)
     } else {

tests/utils/atomWithDefault.test.tsx

@@ -0,0 +1,85 @@
+import React, { Fragment, Suspense } from 'react'
+import { fireEvent, render } from '@testing-library/react'
+import { Provider as ProviderOrig, atom, useAtom } from '../../src/index'
+import { atomWithDefault } from '../../src/utils'
+
+const Provider = process.env.PROVIDER_LESS_MODE ? Fragment : ProviderOrig
+
+it('simple sync get default', async () => {
+  const count1Atom = atom(1)
+  const count2Atom = atomWithDefault((get) => get(count1Atom) * 2)
+
+  const Counter: React.FC = () => {
+    const [count1, setCount1] = useAtom(count1Atom)
+    const [count2, setCount2] = useAtom(count2Atom)
+    return (
+      <>
+        <div>
+          count1: {count1}, count2: {count2}
+        </div>
+        <button onClick={() => setCount1((c) => c + 1)}>button1</button>
+        <button onClick={() => setCount2((c) => c + 1)}>button2</button>
+      </>
+    )
+  }
+
+  const { findByText, getByText } = render(
+    <Provider>
+      <Counter />
+    </Provider>
+  )
+
+  await findByText('count1: 1, count2: 2')
+
+  fireEvent.click(getByText('button1'))
+  await findByText('count1: 2, count2: 4')
+
+  fireEvent.click(getByText('button2'))
+  await findByText('count1: 2, count2: 5')
+
+  fireEvent.click(getByText('button1'))
+  await findByText('count1: 3, count2: 5')
+})
+
+it('simple async get default', async () => {
+  const count1Atom = atom(1)
+  const count2Atom = atomWithDefault(async (get) => {
+    await new Promise((r) => setTimeout(r, 10))
+    return get(count1Atom) * 2
+  })
+
+  const Counter: React.FC = () => {
+    const [count1, setCount1] = useAtom(count1Atom)
+    const [count2, setCount2] = useAtom(count2Atom)
+    return (
+      <>
+        <div>
+          count1: {count1}, count2: {count2}
+        </div>
+        <button onClick={() => setCount1((c) => c + 1)}>button1</button>
+        <button onClick={() => setCount2((c) => c + 1)}>button2</button>
+      </>
+    )
+  }
+
+  const { findByText, getByText } = render(
+    <Provider>
+      <Suspense fallback="loading">
+        <Counter />
+      </Suspense>
+    </Provider>
+  )
+
+  await findByText('loading')
+  await findByText('count1: 1, count2: 2')
+
+  fireEvent.click(getByText('button1'))
+  await findByText('loading')
+  await findByText('count1: 2, count2: 4')
+
+  fireEvent.click(getByText('button2'))
+  await findByText('count1: 2, count2: 5')
+
+  fireEvent.click(getByText('button1'))
+  await findByText('count1: 3, count2: 5')
+})