add some tests, add doc comment

Author: phryneas

docs/rules/extensions.md

@@ -106,6 +106,14 @@ import express from 'express/index';
 import * as path from 'path';
 ```
 
+The following patterns are considered problems when the configuration is set to "never" and the option "checkTypeImports" is set to `true`:
+
+```js
+import type { Foo } from './foo.ts';
+
+export type { Foo } from './foo.ts';
+```
+
 The following patterns are considered problems when configuration set to "always":
 
 ```js
@@ -169,7 +177,7 @@ import express from 'express';
 import foo from '@/foo';
 ```
 
-The following patterns are considered problems when the option "checkTypeImports" is set to `true`:
+The following patterns are considered problems when the configuration is set to "always" and the option "checkTypeImports" is set to `true`:
 
 ```js
 import type { Foo } from './foo';

tests/src/rules/extensions.js

@@ -3,6 +3,15 @@ import rule from 'rules/extensions';
 import { getTSParsers, test, testFilePath, parsers } from '../utils';
 
 const ruleTester = new RuleTester();
+const ruleTesterWithTypeScriptImports = new RuleTester({
+  settings: {
+    'import/resolver': {
+      typescript: {
+        alwaysTryTypes: true,
+      },
+    },
+  },
+});
 
 ruleTester.run('extensions', rule, {
   valid: [
@@ -660,5 +669,45 @@ describe('TypeScript', () => {
           }),
         ],
       });
+      ruleTesterWithTypeScriptImports.run(`${parser}: (with TS resolver) extensions are enforced for type imports/export when checkTypeImports is set`, rule, {
+        valid: [
+          test({
+            code: 'import type { MyType } from "./typescript-declare.ts";',
+            options: [
+              'always',
+              { checkTypeImports: true },
+            ],
+            parser,
+          }),
+          test({
+            code: 'export type { MyType } from "./typescript-declare.ts";',
+            options: [
+              'always',
+              { checkTypeImports: true },
+            ],
+            parser,
+          }),
+        ],
+        invalid: [
+          test({
+            code: 'import type { MyType } from "./typescript-declare";',
+            errors: ['Missing file extension "ts" for "./typescript-declare"'],
+            options: [
+              'always',
+              { checkTypeImports: true },
+            ],
+            parser,
+          }),
+          test({
+            code: 'export type { MyType } from "./typescript-declare";',
+            errors: ['Missing file extension "ts" for "./typescript-declare"'],
+            options: [
+              'always',
+              { checkTypeImports: true },
+            ],
+            parser,
+          }),
+        ],
+      });
     });
 });