fix(base-controller): add TypeScript declaration file for legacy module resolution

packages/base-controller/CHANGELOG.md

@@ -7,6 +7,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+
+- Fix TypeScript module resolution for `/next` subpath export with legacy resolution mode ([#6915](https://github.com/MetaMask/core/pull/6915))
+  - Added `next.d.ts` file to enable imports like `import { BaseController } from '@metamask/base-controller/next'` to work with both legacy TypeScript module resolution and Node16/NodeNext resolution modes
+  - Previously, this import pattern only worked with Node16/NodeNext resolution which uses the `exports` field in package.json
+
 ## [8.4.1]
 
 ### Changed

packages/base-controller/next.d.ts

@@ -0,0 +1,7 @@
+// Type definitions for compatibility with legacy TypeScript module resolution.
+// Node16/NodeNext resolution uses the "exports" field in package.json,
+// but legacy resolution needs explicit .d.ts files at the import path.
+
+// Re-export all types and values from the CommonJS build
+// eslint-disable-next-line import-x/no-useless-path-segments, import-x/extensions
+export * from './dist/next/index.cjs';

packages/base-controller/package.json

@@ -42,7 +42,8 @@
   "types": "./dist/index.d.cts",
   "files": [
     "dist/",
-    "next.js"
+    "next.js",
+    "next.d.ts"
   ],
   "scripts": {
     "build": "ts-bridge --project tsconfig.build.json --verbose --clean --no-references",