Require ECMAScript private class members

common/changes/@rushstack/eslint-config/native-private-fields_2026-08-18-12-00-00.json

@@ -0,0 +1,9 @@
+{
+  "changes": [
+    {
+      "packageName": "@rushstack/eslint-config",
+      "comment": "Enable the rule that prefers ECMAScript private class members.",
+      "type": "minor"
+    }
+  ]
+}

common/changes/@rushstack/eslint-plugin/native-private-fields_2026-08-18-12-00-00.json

@@ -0,0 +1,9 @@
+{
+  "changes": [
+    {
+      "packageName": "@rushstack/eslint-plugin",
+      "comment": "Add a rule that requires ECMAScript private syntax for class fields, methods, and accessors.",
+      "type": "minor"
+    }
+  ]
+}

eslint/eslint-config/flat/profile/_common.js

@@ -220,6 +220,9 @@ const commonConfig = [
       // RATIONALE:         See the @rushstack/eslint-plugin documentation
       '@rushstack/no-new-null': 'warn',
 
+      // RATIONALE:         See the @rushstack/eslint-plugin documentation
+      '@rushstack/prefer-ecmascript-private-members': 'warn',
+
       // RATIONALE:         See the @rushstack/eslint-plugin documentation
       '@rushstack/typedef-var': 'warn',
 

eslint/eslint-config/profile/_common.js

@@ -236,6 +236,9 @@ function buildRules(profile) {
           // RATIONALE:         See the @rushstack/eslint-plugin documentation
           '@rushstack/no-new-null': 'warn',
 
+          // RATIONALE:         See the @rushstack/eslint-plugin documentation
+          '@rushstack/prefer-ecmascript-private-members': 'warn',
+
           // RATIONALE:         See the @rushstack/eslint-plugin documentation
           '@rushstack/typedef-var': 'warn',
 

eslint/eslint-plugin/README.md

@@ -425,6 +425,41 @@ enum E {
 let e: E._PrivateMember = E._PrivateMember; // okay, because _PrivateMember is declared by E
 ```
 
+## `@rushstack/prefer-ecmascript-private-members`
+
+Require ECMAScript private syntax for fields, methods, and accessors declared with TypeScript's `private`
+modifier.
+
+#### Rule Details
+
+ECMAScript `#` members provide runtime privacy. TypeScript's `private` modifier is erased during compilation,
+allowing the member to be accessed through JavaScript, bracket notation, or type assertions.
+
+This rule applies to class fields, methods, and accessors. Private constructors and constructor parameter
+properties are not affected. The rule does not provide an autofix because converting a member requires updating
+every reference and may change runtime behavior for reflection or objects created without invoking the
+constructor.
+
+#### Examples
+
+The following pattern is considered a problem:
+
+```ts
+class Example {
+  private value: string = ''; // error
+  private calculate(): number {} // error
+}
+```
+
+The following pattern is NOT considered a problem:
+
+```ts
+class Example {
+  #value: string = '';
+  #calculate(): number {}
+}
+```
+
 ## `@rushstack/normalized-imports`
 
 Require relative import paths to be written in a normalized minimal form and autofix unnecessary directory traversals.

eslint/eslint-plugin/src/index.ts

@@ -14,6 +14,7 @@ import { normalizedImportsRule } from './normalized-imports';
 import { typedefVar } from './typedef-var';
 import { importRequiresChunkNameRule } from './import-requires-chunk-name';
 import { pairReactDomRenderUnmountRule } from './pair-react-dom-render-unmount';
+import { preferEcmascriptPrivateMembersRule } from './prefer-ecmascript-private-members';
 
 interface IPlugin {
   rules: { [ruleName: string]: TSESLint.RuleModule<string, unknown[]> };
@@ -52,7 +53,10 @@ const plugin: IPlugin = {
     'import-requires-chunk-name': importRequiresChunkNameRule,
 
     // Full name: "@rushstack/pair-react-dom-render-unmount"
-    'pair-react-dom-render-unmount': pairReactDomRenderUnmountRule
+    'pair-react-dom-render-unmount': pairReactDomRenderUnmountRule,
+
+    // Full name: "@rushstack/prefer-ecmascript-private-members"
+    'prefer-ecmascript-private-members': preferEcmascriptPrivateMembersRule
   }
 };
 

eslint/eslint-plugin/src/prefer-ecmascript-private-members.ts

@@ -0,0 +1,44 @@
+// Copyright (c) Microsoft Corporation. All rights reserved. Licensed under the MIT license.
+// See LICENSE in the project root for license information.
+
+import type { TSESLint, TSESTree } from '@typescript-eslint/utils';
+
+type MessageIds = 'use-ecmascript-private-member';
+type Options = [];
+
+const preferEcmascriptPrivateMembersRule: TSESLint.RuleModule<MessageIds, Options> = {
+  defaultOptions: [],
+  meta: {
+    type: 'suggestion',
+    messages: {
+      'use-ecmascript-private-member':
+        'Use ECMAScript private syntax ("#member") instead of the TypeScript "private" modifier.'
+    },
+    schema: [],
+    docs: {
+      description: 'Require ECMAScript private syntax for private class fields, methods, and accessors',
+      recommended: 'recommended',
+      url: 'https://www.npmjs.com/package/@rushstack/eslint-plugin'
+    } as TSESLint.RuleMetaDataDocs
+  },
+  create: (context: TSESLint.RuleContext<MessageIds, Options>) => ({
+    PropertyDefinition(node: TSESTree.PropertyDefinition): void {
+      if (node.accessibility === 'private') {
+        context.report({
+          node,
+          messageId: 'use-ecmascript-private-member'
+        });
+      }
+    },
+    MethodDefinition(node: TSESTree.MethodDefinition): void {
+      if (node.accessibility === 'private' && node.kind !== 'constructor') {
+        context.report({
+          node,
+          messageId: 'use-ecmascript-private-member'
+        });
+      }
+    }
+  })
+};
+
+export { preferEcmascriptPrivateMembersRule };

eslint/eslint-plugin/src/test/prefer-ecmascript-private-members.test.ts

@@ -0,0 +1,80 @@
+// Copyright (c) Microsoft Corporation. All rights reserved. Licensed under the MIT license.
+// See LICENSE in the project root for license information.
+
+import type { RuleTester } from '@typescript-eslint/rule-tester';
+
+import { preferEcmascriptPrivateMembersRule } from '../prefer-ecmascript-private-members';
+import { getRuleTesterWithoutProject } from './ruleTester';
+
+const ruleTester: RuleTester = getRuleTesterWithoutProject();
+
+ruleTester.run('prefer-ecmascript-private-members', preferEcmascriptPrivateMembersRule, {
+  invalid: [
+    {
+      code: 'class Example { private value: string = ""; }',
+      errors: [{ messageId: 'use-ecmascript-private-member' }]
+    },
+    {
+      code: 'class Example { private static readonly values: Set<string> = new Set(); }',
+      errors: [{ messageId: 'use-ecmascript-private-member' }]
+    },
+    {
+      code: 'class Example { private optional?: string; private assigned!: string; }',
+      errors: [
+        { messageId: 'use-ecmascript-private-member' },
+        { messageId: 'use-ecmascript-private-member' }
+      ]
+    },
+    {
+      code: 'class Example { declare private value: string; }',
+      errors: [{ messageId: 'use-ecmascript-private-member' }]
+    },
+    {
+      code: 'class Example { private ["value"]: string = ""; }',
+      errors: [{ messageId: 'use-ecmascript-private-member' }]
+    },
+    {
+      code: 'class Example { private calculate(): number { return 1; } }',
+      errors: [{ messageId: 'use-ecmascript-private-member' }]
+    },
+    {
+      code: [
+        'class Example {',
+        '  private get value(): string { return ""; }',
+        '  private set value(value: string) {}',
+        '}'
+      ].join('\n'),
+      errors: [
+        { messageId: 'use-ecmascript-private-member' },
+        { messageId: 'use-ecmascript-private-member' }
+      ]
+    }
+  ],
+  valid: [
+    {
+      code: 'class Example { #value: string = ""; static #values: Set<string> = new Set(); }'
+    },
+    {
+      code: [
+        'class Example {',
+        '  #calculate(): number { return 1; }',
+        '  get #value(): string { return ""; }',
+        '  set #value(value: string) {}',
+        '}'
+      ].join('\n')
+    },
+    {
+      code: 'class Example { public value: string = ""; protected otherValue: string = ""; }'
+    },
+    {
+      code: [
+        'class Example {',
+        '  public constructor(private readonly parameter: string) {}',
+        '}'
+      ].join('\n')
+    },
+    {
+      code: 'class Example { private constructor() {} }'
+    }
+  ]
+});