From 5521527cf44cdbc121023573bced9d4bebe416b2 Mon Sep 17 00:00:00 2001
From: Michel Weststrate <mweststrate@fb.com>
Date: Fri, 24 Jul 2020 17:19:21 +0100
Subject: [PATCH] fix: make sure changing an undefined value to undefined is
 not picked up as change. Fixes #646

---
 __tests__/regressions.js | 20 ++++++++++++++++++++
 src/core/proxy.ts        |  7 ++++++-
 2 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/__tests__/regressions.js b/__tests__/regressions.js
index 9c5bae33..e0fc30b1 100644
--- a/__tests__/regressions.js
+++ b/__tests__/regressions.js
@@ -78,5 +78,25 @@ function runBaseTest(name, useProxies, autoFreeze, useListener) {
 				x: 1
 			})
 		})
+
+		test("#646 setting undefined field to undefined should not create new result", () => {
+			const foo = {
+				bar: undefined
+			}
+			const foo2 = produce(foo, draft => {
+				draft.bar = undefined
+			})
+			expect(foo2).toBe(foo)
+		})
+
+		test("#646 -2 setting undefined field to undefined should not create new result", () => {
+			const foo = {}
+			const foo2 = produce(foo, draft => {
+				draft.bar = undefined
+			})
+			expect(foo2).not.toBe(foo)
+			expect(foo).toEqual({})
+			expect(foo2).toEqual({bar: undefined})
+		})
 	})
 }
diff --git a/src/core/proxy.ts b/src/core/proxy.ts
index 94b89f85..13140a5c 100644
--- a/src/core/proxy.ts
+++ b/src/core/proxy.ts
@@ -139,7 +139,12 @@ export const objectTraps: ProxyHandler<ProxyState> = {
 		}
 		state.assigned_[prop] = true
 		if (!state.modified_) {
-			if (is(value, peek(latest(state), prop)) && value !== undefined)
+			// the last check is because we need to be able to distinguish setting a non-existig to undefined (which is a change)
+			// from setting an existing property with value undefined to undefined (which is not a change)
+			if (
+				is(value, peek(latest(state), prop)) &&
+				(value !== undefined || has(state.base_, prop))
+			)
 				return true
 			prepareCopy(state)
 			markChanged(state)