fix(eslint-plugin): support arbitrary extensions in definition files (#10957)

* support arbitrary extensions in definition files

* updated test case

* updated test and refactoring

Author: dbarabashh

packages/eslint-plugin/src/util/misc.ts

@@ -25,7 +25,8 @@ export function isDefinitionFile(fileName: string): boolean {
       return true;
     }
   }
-  return false;
+
+  return /\.d\.(ts|cts|mts|.*\.ts)$/.test(lowerFileName);
 }
 
 /**

packages/eslint-plugin/tests/util/misc.test.ts

@@ -1,5 +1,28 @@
 import * as misc from '../../src/util/misc';
 
+describe('isDefinitionFile', () => {
+  it.each([['index.d.ts'], ['module.d.cts'], ['package.d.mts']])(
+    'returns true for standard definition file: %s',
+    filename => {
+      expect(misc.isDefinitionFile(filename)).toBe(true);
+    },
+  );
+
+  it.each([['styles.d.css.ts'], ['component.d.vue.ts'], ['env.d.node.ts']])(
+    'returns true for arbitrary extension definition file: %s',
+    filename => {
+      expect(misc.isDefinitionFile(filename)).toBe(true);
+    },
+  );
+
+  it.each([['index.ts'], ['app.tsx'], ['styles.css.ts'], ['vite.config.ts']])(
+    'returns false for non definition file: %s',
+    filename => {
+      expect(misc.isDefinitionFile(filename)).toBe(false);
+    },
+  );
+});
+
 describe('formatWordList', () => {
   it('can format with no words', () => {
     expect(misc.formatWordList([])).toBe('');