fix: add undefined initial value to primitive Atom definition (#2668)

Co-authored-by: Daishi Kato <dai-shi@users.noreply.github.com>

src/vanilla/atom.ts

@@ -81,13 +81,17 @@ export function atom<Value, Args extends unknown[], Result>(
   write: Write<Args, Result>,
 ): WritableAtom<Value, Args, Result> & WithInitialValue<Value>
 
+// primitive atom without initial value
+export function atom<Value>(): PrimitiveAtom<Value | undefined> &
+  WithInitialValue<Value | undefined>
+
 // primitive atom
 export function atom<Value>(
   initialValue: Value,
 ): PrimitiveAtom<Value> & WithInitialValue<Value>
 
 export function atom<Value, Args extends unknown[], Result>(
-  read: Value | Read<Value, SetAtom<Args, Result>>,
+  read?: Value | Read<Value, SetAtom<Args, Result>>,
   write?: Write<Args, Result>,
 ) {
   const key = `atom${++keyCount}`
@@ -97,7 +101,7 @@ export function atom<Value, Args extends unknown[], Result>(
         ? key + ':' + this.debugLabel
         : key
     },
-  } as WritableAtom<Value, Args, Result> & { init?: Value }
+  } as WritableAtom<Value, Args, Result> & { init?: Value | undefined }
   if (typeof read === 'function') {
     config.read = read as Read<Value, SetAtom<Args, Result>>
   } else {

tests/vanilla/types.test.tsx

@@ -1,4 +1,5 @@
 import { expectType } from 'ts-expect'
+import type { TypeOf } from 'ts-expect'
 import { it } from 'vitest'
 import { atom } from 'jotai/vanilla'
 import type {
@@ -15,6 +16,15 @@ it('atom() should return the correct types', () => {
     // primitive atom
     const primitiveAtom = atom(0)
     expectType<PrimitiveAtom<number>>(primitiveAtom)
+    expectType<TypeOf<PrimitiveAtom<number>, typeof primitiveAtom>>(true)
+    expectType<TypeOf<PrimitiveAtom<number | undefined>, typeof primitiveAtom>>(
+      false,
+    )
+
+    // primitive atom without initial value
+    const primitiveWithoutInitialAtom = atom<number | undefined>()
+    expectType<PrimitiveAtom<number | undefined>>(primitiveWithoutInitialAtom)
+    expectType<PrimitiveAtom<undefined>>(atom())
 
     // read-only derived atom
     const readonlyDerivedAtom = atom((get) => get(primitiveAtom) * 2)