From 7f8cceb79d86179b494b367ea6698ed2234ca9cd Mon Sep 17 00:00:00 2001
From: rtritto <ariele.1994@gmail.com>
Date: Tue, 23 Jul 2024 10:30:49 +0200
Subject: [PATCH] fix: add undefined initial value to primitive Atom definition

---
 src/vanilla/atom.ts          |  8 ++++++--
 tests/vanilla/types.test.tsx | 10 ++++++++++
 2 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/src/vanilla/atom.ts b/src/vanilla/atom.ts
index b2cfcba268..c4d5b513e5 100644
--- a/src/vanilla/atom.ts
+++ b/src/vanilla/atom.ts
@@ -81,19 +81,23 @@ 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}`
   const config = {
     toString: () => 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 {
diff --git a/tests/vanilla/types.test.tsx b/tests/vanilla/types.test.tsx
index e0744b887b..3c89725bc7 100644
--- a/tests/vanilla/types.test.tsx
+++ b/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)