sindresorhus
diff --git a/‎docs/rules/no-undeclared-class-members.md‎
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diff --git a/‎readme.md‎
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diff --git a/‎rules/index.js‎
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@@ -0,0 +1,70 @@
+# no-undeclared-class-members
+
+📝 Require class members to be declared.
+
+💼🚫 This rule is enabled in the ✅ `recommended` [config](https://github.com/sindresorhus/eslint-plugin-unicorn#recommended-config). This rule is _disabled_ in the ☑️ `unopinionated` [config](https://github.com/sindresorhus/eslint-plugin-unicorn#recommended-config).
+
+💡 This rule is manually fixable by [editor suggestions](https://eslint.org/docs/latest/use/core-concepts#rule-suggestions).
+
+<!-- end auto-generated rule header -->
+<!-- Do not manually modify this header. Run: `npm run fix:eslint-docs` -->
+
+Class members should be declared when they are used through `this`. Declaring fields, methods, getters, and setters in the class body makes typos visible and gives readers the class shape upfront.
+
+Constructor assignments like `this.name = name` are treated as declarations. Classes with `extends` are ignored because the accessed member may come from a superclass. Static `this`, private members, computed member access, and dynamic initialization patterns like `Object.assign(this, data)` are unsupported.
+
+Editor suggestions are offered for simple non-constructor assignments like `this.name = name` when a declaration can be inserted cleanly. Reads, calls, compound assignments, updates, and constructor assignments are reported without a suggestion.
+
+## Examples
+
+```js
+// ❌
+class Foo {
+	getName() {
+		return this.name;
+	}
+}
+
+// ✅
+class Foo {
+	name;
+
+	getName() {
+		return this.name;
+	}
+}
+```
+
+```js
+// ❌
+class Foo {
+	callName() {
+		this.name();
+	}
+}
+
+// ✅
+class Foo {
+	name() {}
+
+	callName() {
+		this.name();
+	}
+}
+```
+
+```js
+// ✅
+class Foo {
+	constructor(name) {
+		this.name = name;
+	}
+
+	getName() {
+		return this.name;
+	}
+}
+```
+
+> [!NOTE]
+> Declaring a class field like `name;` creates an own property with the value `undefined` on every instance before any method runs. This can change observable behavior, for example `Object.hasOwn(instance, 'name')`, so this rule provides editor suggestions instead of an autofix.
@@ -145,6 +145,7 @@ export default [
 | [no-this-assignment](docs/rules/no-this-assignment.md)                                                 | Disallow assigning `this` to a variable.                                                                                                                                                                          | ✅ ☑️ |    |    |
 | [no-this-outside-of-class](docs/rules/no-this-outside-of-class.md)                                     | Disallow `this` outside of classes.                                                                                                                                                                               | ✅    |    |    |
 | [no-typeof-undefined](docs/rules/no-typeof-undefined.md)                                               | Disallow comparing `undefined` using `typeof`.                                                                                                                                                                    | ✅ ☑️ | 🔧 | 💡 |
+| [no-undeclared-class-members](docs/rules/no-undeclared-class-members.md)                               | Require class members to be declared.                                                                                                                                                                             | ✅    |    | 💡 |
 | [no-unnecessary-array-flat-depth](docs/rules/no-unnecessary-array-flat-depth.md)                       | Disallow using `1` as the `depth` argument of `Array#flat()`.                                                                                                                                                     | ✅ ☑️ | 🔧 |    |
 | [no-unnecessary-array-splice-count](docs/rules/no-unnecessary-array-splice-count.md)                   | Disallow using `.length` or `Infinity` as the `deleteCount` or `skipCount` argument of `Array#{splice,toSpliced}()`.                                                                                              | ✅ ☑️ | 🔧 |    |
 | [no-unnecessary-await](docs/rules/no-unnecessary-await.md)                                             | Disallow awaiting non-promise values.                                                                                                                                                                             | ✅ ☑️ | 🔧 |    |
 
@@ -84,6 +84,7 @@ export {default as 'no-thenable'} from './no-thenable.js';
 export {default as 'no-this-assignment'} from './no-this-assignment.js';
 export {default as 'no-this-outside-of-class'} from './no-this-outside-of-class.js';
 export {default as 'no-typeof-undefined'} from './no-typeof-undefined.js';
+export {default as 'no-undeclared-class-members'} from './no-undeclared-class-members.js';
 export {default as 'no-unnecessary-array-flat-depth'} from './no-unnecessary-array-flat-depth.js';
 export {default as 'no-unnecessary-array-splice-count'} from './no-unnecessary-array-splice-count.js';
 export {default as 'no-unnecessary-await'} from './no-unnecessary-await.js';