From 20d21ca4268853c85d203f1270c77c40922a2052 Mon Sep 17 00:00:00 2001
From: Taj Pereira <tajpereira@gmail.com>
Date: Sat, 1 Jun 2024 06:17:44 +0930
Subject: [PATCH 1/3] build: set useDefineForClassFields to false

---
 .changeset/tiny-boxes-play.md | 9 +++++++++
 tsconfig.json                 | 2 +-
 2 files changed, 10 insertions(+), 1 deletion(-)
 create mode 100644 .changeset/tiny-boxes-play.md

diff --git a/.changeset/tiny-boxes-play.md b/.changeset/tiny-boxes-play.md
new file mode 100644
index 0000000000..4bcfec3111
--- /dev/null
+++ b/.changeset/tiny-boxes-play.md
@@ -0,0 +1,9 @@
+---
+"mobx": patch
+"eslint-plugin-mobx": patch
+"mobx-react": patch
+"mobx-react-lite": patch
+"mobx-undecorate": patch
+---
+
+In TypeScript transpilation, disable useDefineForClassFields to reduce memory requirements of classes having optional members.
diff --git a/tsconfig.json b/tsconfig.json
index c0c18f028f..32fa6607d9 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -16,7 +16,7 @@
         "noImplicitThis": false,
         "noEmit": true,
         "experimentalDecorators": true,
-        "useDefineForClassFields": true,
+        "useDefineForClassFields": false,
         "jsx": "react",
         "esModuleInterop": true
     },

From 18c82b96dbd0fa53a16353b6627695f20f399609 Mon Sep 17 00:00:00 2001
From: Taj Pereira <tajpereira@gmail.com>
Date: Sat, 1 Jun 2024 10:35:40 +0930
Subject: [PATCH 2/3] fix: enable for tests

---
 tsconfig.json      | 1 +
 tsconfig.test.json | 4 +++-
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/tsconfig.json b/tsconfig.json
index 32fa6607d9..7a26c1bb72 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -16,6 +16,7 @@
         "noImplicitThis": false,
         "noEmit": true,
         "experimentalDecorators": true,
+        // See: https://github.com/mobxjs/mobx/pull/3884
         "useDefineForClassFields": false,
         "jsx": "react",
         "esModuleInterop": true
diff --git a/tsconfig.test.json b/tsconfig.test.json
index f8e4f8c916..0bfdfaf815 100644
--- a/tsconfig.test.json
+++ b/tsconfig.test.json
@@ -3,6 +3,8 @@
     "compilerOptions": {
         // "module": "commonjs",
         "allowJs": true,
-        "noUnusedLocals": false
+        "noUnusedLocals": false,
+        // See: https://github.com/mobxjs/mobx/pull/3884
+        "useDefineForClassFields": true
     }
 }

From 3d894152a47786a38d95a4c0ce48e1e1cb4d8221 Mon Sep 17 00:00:00 2001
From: Taj Pereira <taj@canva.com>
Date: Sun, 2 Jun 2024 08:16:05 +0930
Subject: [PATCH 3/3] Update .changeset/tiny-boxes-play.md

---
 .changeset/tiny-boxes-play.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/.changeset/tiny-boxes-play.md b/.changeset/tiny-boxes-play.md
index 4bcfec3111..d0bd3fd006 100644
--- a/.changeset/tiny-boxes-play.md
+++ b/.changeset/tiny-boxes-play.md
@@ -6,4 +6,4 @@
 "mobx-undecorate": patch
 ---
 
-In TypeScript transpilation, disable useDefineForClassFields to reduce memory requirements of classes having optional members.
+Disable `useDefineForClassFields` in `tsconfig.json` to reduce memory footprint and improve performance of class instances with undefined and unused members.