From 11e5c714348f3c47476c23b0c1e0b6a2f2f75369 Mon Sep 17 00:00:00 2001
From: Dawid Rusnak <rusnak@evidentid.com>
Date: Wed, 9 Feb 2022 13:26:49 +0100
Subject: [PATCH] Inform about `useDefineForClassFields` breaking change in TS
 >=4.3

---
 Breaking-Changes.md | 46 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 46 insertions(+)

diff --git a/Breaking-Changes.md b/Breaking-Changes.md
index 65e03910..45dcd4f6 100644
--- a/Breaking-Changes.md
+++ b/Breaking-Changes.md
@@ -249,6 +249,52 @@ enum E {
 
 For more details, [see the original change](https://github.com/microsoft/TypeScript/pull/42472)
 
+## Class fields while extending classes
+
+TypeScript 4.3 introduces new `useDefineForClassFields` compiler option, which defaults to `true`.
+
+The class' fields initialization will be included then.
+
+```ts
+class Example {
+    private abc: string;
+}
+```
+
+Becomes:
+
+```js
+// TypeScript <4.3 or "useDefineForClassFields": false
+class Example {
+}
+
+// TypeScript >=4.3 default behavior - "useDefineForClassFields": true
+class Example {
+    abc;
+}
+```
+
+It may introduce the problem when extending classes - after `super` constructor will be called, all child class properties will be reinitialized.
+It may lead to unexpected behavior, where the value was set during super class constructor, but it's reset later.
+
+```ts
+class Parent {
+    public value: string | null;
+
+    public constructor() {
+        this.value = "test value";
+    }
+}
+
+class Child extends Parent {
+    public value!: string;
+}
+
+console.log(new Child().value);
+// for `useDefineForClassFields: false` -> "test value"
+// for `useDefineForClassFields: true`  -> undefined
+```
+
 # TypeScript 4.2
 
 ## `noImplicitAny` Errors Apply to Loose `yield` Expressions