fix: make extensions rule handle .d.ts correctly (#468)

Co-authored-by: Claude (Opus 4.6) <noreply@anthropic.com>
Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>

Author: 3 people

.changeset/fix-extensions-dts.md

@@ -0,0 +1,5 @@
+---
+"eslint-plugin-import-x": patch
+---
+
+Make `extensions` handle `.d.ts` correctly

src/rules/extensions.ts

@@ -16,6 +16,8 @@ import {
   stringifyPath,
 } from '../utils/index.js'
 
+const dtsRe = /\.d\.[cm]?ts$/
+
 const modifierValues = ['always', 'ignorePackages', 'never'] as const
 
 const modifierSchema = {
@@ -328,9 +330,15 @@ export default createRule<Options, MessageId>({
 
         const resolvedPath = resolve(importPath, context)
 
-        // get extension from resolved path, if possible.
-        // for unresolved, use source value.
-        const extension = path.extname(resolvedPath || importPath).slice(1)
+        // get extension from resolved path, or source value if unresolved.
+        // for .d.ts/.d.mts/.d.cts, use the import path extension instead.
+        const extension = path
+          .extname(
+            resolvedPath && dtsRe.test(resolvedPath)
+              ? importPath
+              : resolvedPath || importPath,
+          )
+          .slice(1)
 
         // determine if this is a module
         const isPackage =

test/fixtures/dist-import/index.d.ts

@@ -0,0 +1 @@
+export declare const foo: string

test/fixtures/dist-import/index.js

@@ -0,0 +1 @@
+export const foo = 'bar'

test/rules/extensions.spec.ts

@@ -1249,6 +1249,12 @@ describe('TypeScript', () => {
         options: ['always', { checkTypeImports: true }],
       }),
 
+      // .js import that resolves to .d.ts should not demand .ts extension
+      tValid({
+        code: 'import { foo } from "./dist-import/index.js";',
+        options: ['always'],
+      }),
+
       // pathGroupOverrides: no patterns match good bespoke specifiers
       tValid({
         code: `