fix: #807 new undefined properties should end up in result object

__tests__/regressions.js

@@ -239,5 +239,17 @@ function runBaseTest(name, useProxies, autoFreeze, useListener) {
 				bar: {x: 3}
 			})
 		})
+
+		test("#807 new undefined member not stored", () => {
+			const state = {}
+			const newState = produce(state, draft => {
+				draft.baz = undefined
+			})
+			expect(state).not.toBe(newState)
+			expect(Object.hasOwnProperty.call(newState, "baz")).toBe(true)
+			expect(newState).toEqual({
+				baz: undefined
+			})
+		})
 	})
 }

src/core/proxy.ts

@@ -157,7 +157,14 @@ export const objectTraps: ProxyHandler<ProxyState> = {
 			markChanged(state)
 		}
 
-		if (state.copy_![prop] === value && typeof value !== "number") return true
+		if (
+			state.copy_![prop] === value &&
+			// special case: NaN
+			typeof value !== "number" &&
+			// special case: handle new props with value 'undefined'
+			(value !== undefined || prop in state.copy_)
+		)
+			return true
 
 		// @ts-ignore
 		state.copy_![prop] = value