From df4b09f06ec5af8451b59543f33b606271a75ef3 Mon Sep 17 00:00:00 2001
From: Bobbie Goede <bobbiegoede@gmail.com>
Date: Fri, 13 Sep 2024 11:52:07 +0200
Subject: [PATCH] fix: messages `deepCopy` mutates `src` arguments

---
 packages/shared/src/messages.ts       | 14 +++++---
 packages/shared/test/messages.test.ts | 50 +++++++++++++++++++++++++++
 2 files changed, 60 insertions(+), 4 deletions(-)
 create mode 100644 packages/shared/test/messages.test.ts

diff --git a/packages/shared/src/messages.ts b/packages/shared/src/messages.ts
index d657eed02..719a0361a 100644
--- a/packages/shared/src/messages.ts
+++ b/packages/shared/src/messages.ts
@@ -13,14 +13,20 @@ export function deepCopy(src: any, des: any): void {
     const { src, des } = stack.pop()!
 
     Object.keys(src).forEach(key => {
-      if (isNotObjectOrIsArray(src[key]) || isNotObjectOrIsArray(des[key])) {
+      // if src[key] is an object/array, set des[key]
+      // to empty object/array to prevent setting by reference
+      if (isObject(src[key]) && !isObject(des[key])) {
+        des[key] = Array.isArray(src[key]) ? [] : {}
+      }
+
+      if (isObject(des[key]) && isObject(src[key])) {
+        // src[key] and des[key] are both objects, merge them
+        stack.push({ src: src[key], des: des[key] })
+      } else {
         // replace with src[key] when:
         // src[key] or des[key] is not an object, or
         // src[key] or des[key] is an array
         des[key] = src[key]
-      } else {
-        // src[key] and des[key] are both objects, merge them
-        stack.push({ src: src[key], des: des[key] })
       }
     })
   }
diff --git a/packages/shared/test/messages.test.ts b/packages/shared/test/messages.test.ts
new file mode 100644
index 000000000..1380dd10f
--- /dev/null
+++ b/packages/shared/test/messages.test.ts
@@ -0,0 +1,50 @@
+import { deepCopy } from '../src/index'
+
+test('deepCopy merges without mutating src argument', () => {
+  const msg1 = {
+    hello: 'Greetings',
+    about: {
+      title: 'About us'
+    },
+    overwritten: 'Original text',
+    fruit: [{ name: 'Apple' }]
+  }
+  const copy1 = structuredClone(msg1)
+
+  const msg2 = {
+    bye: 'Goodbye',
+    about: {
+      content: 'Some text'
+    },
+    overwritten: 'New text',
+    fruit: [{ name: 'Strawberry' }],
+    // @ts-ignore
+    car: ({ plural }) => plural(['car', 'cars'])
+  }
+
+  const merged = {}
+
+  deepCopy(msg1, merged)
+  deepCopy(msg2, merged)
+
+  expect(merged).toMatchInlineSnapshot(`
+    {
+      "about": {
+        "content": "Some text",
+        "title": "About us",
+      },
+      "bye": "Goodbye",
+      "car": [Function],
+      "fruit": [
+        {
+          "name": "Strawberry",
+        },
+      ],
+      "hello": "Greetings",
+      "overwritten": "New text",
+    }
+  `)
+
+  // should not mutate source object
+  expect(msg1).toStrictEqual(copy1)
+})