fix: add undefined initial value to primitive Atom definition

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 default 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 {

tests/vanilla/basic.test.tsx

@@ -2,6 +2,8 @@ import { expect, it } from 'vitest'
 import { atom } from 'jotai/vanilla'
 
 it('creates atoms', () => {
+  // primitive atom without default value
+  const undefinedAtom = atom()
   // primitive atom
   const countAtom = atom(0)
   const anotherCountAtom = atom(1)
@@ -20,12 +22,19 @@ it('creates atoms', () => {
     set(countAtom, get(countAtom) - 1)
   })
   expect({
+    undefinedAtom,
     countAtom,
     doubledCountAtom,
     sumCountAtom,
     decrementCountAtom,
   }).toMatchInlineSnapshot(`
     {
+      "undefinedAtom": {
+        "init": undefined,
+        "read": [Function],
+        "toString": [Function],
+        "write": [Function],
+      },
       "countAtom": {
         "init": 0,
         "read": [Function],