fix: preserve assignments that shadow inherited values

__tests__/regressions.js

@@ -274,6 +274,32 @@ function runBaseTest(name, useProxies, autoFreeze, useListener) {
 			])
 		})
 
+		for (const modified of [false, true]) {
+			test.each(["default", NaN])(
+				`assigning an inherited value creates an own property (modified: ${modified}, value: %s)`,
+				value => {
+					const proto = {[immerable]: true, name: value}
+					const state = Object.create(proto)
+
+					const [newState, patches, inversePatches] = produceWithPatches(
+						state,
+						draft => {
+							if (modified) draft.changed = true
+							draft.name = value
+						}
+					)
+
+					expect(newState).not.toBe(state)
+					expect(Object.hasOwnProperty.call(state, "name")).toBe(false)
+					expect(Object.hasOwnProperty.call(newState, "name")).toBe(true)
+					expect(newState.name).toBe(value)
+					expect(Object.getPrototypeOf(newState)).toBe(proto)
+					expect(patches).toContainEqual({op: "add", path: ["name"], value})
+					expect(inversePatches).toContainEqual({op: "remove", path: ["name"]})
+				}
+			)
+		}
+
 		test("Nested and chained produce calls throw 'Cannot perform 'get' on a proxy that has been revoked' error", () => {
 			const state = {
 				foo: {

src/core/proxy.ts

@@ -179,8 +179,7 @@ export const objectTraps: ProxyHandler<ProxyState> = {
 			return true
 		}
 		if (!state.modified_) {
-			// the last check is because we need to be able to distinguish setting a non-existing to undefined (which is a change)
-			// from setting an existing property with value undefined to undefined (which is not a change)
+			// Assigning an inherited value creates an own property, even when the value is unchanged.
 			const current = peek(latest(state), prop)
 			// special case, if we assigning the original value to a draft, we can ignore the assignment
 			const currentState: ProxyObjectState = current?.[DRAFT_STATE]
@@ -189,21 +188,16 @@ export const objectTraps: ProxyHandler<ProxyState> = {
 				state.assigned_!.set(prop, false)
 				return true
 			}
-			if (
-				is(value, current) &&
-				(value !== undefined || has(state.base_, prop, state.type_))
-			)
-				return true
+			if (is(value, current) && has(state.base_, prop, state.type_)) return true
 			prepareCopy(state)
 			markChanged(state)
 		}
 
 		if (
-			(state.copy_![prop] === value &&
-				// special case: handle new props with value 'undefined'
-				(value !== undefined || has(state.copy_, prop, state.type_))) ||
-			// special case: NaN
-			(Number.isNaN(value) && Number.isNaN(state.copy_![prop]))
+			(state.copy_![prop] === value ||
+				// special case: NaN
+				(Number.isNaN(value) && Number.isNaN(state.copy_![prop]))) &&
+			has(state.copy_, prop, state.type_)
 		)
 			return true