fix: support .mts and .cts out of the box

packages/typescript/src/index.ts

@@ -3,7 +3,7 @@ import * as path from 'path';
 import { createFilter } from '@rollup/pluginutils';
 
 import { Plugin, RollupOptions, SourceDescription } from 'rollup';
-import type { Watch } from 'typescript';
+import type { ResolvedModuleFull, Watch } from 'typescript';
 
 import { RollupTypescriptOptions } from '../types';
 
@@ -17,6 +17,11 @@ import { preflight } from './preflight';
 import createWatchProgram, { WatchProgramHelper } from './watchProgram';
 import TSCache from './tscache';
 
+/**
+ * The glob pattern used to test if a given file extension corresponds to a TypeScript file.
+ */
+const defaultTsExtensionsGlob = '?(m|c)ts+(|x)';
+
 export default function typescript(options: RollupTypescriptOptions = {}): Plugin {
   const {
     cacheDir,
@@ -36,9 +41,13 @@ export default function typescript(options: RollupTypescriptOptions = {}): Plugi
   const watchProgramHelper = new WatchProgramHelper();
 
   const parsedOptions = parseTypescriptConfig(ts, tsconfig, compilerOptions, noForceEmit);
-  const filter = createFilter(include || ['*.ts+(|x)', '**/*.ts+(|x)'], exclude, {
-    resolve: filterRoot ?? parsedOptions.options.rootDir
-  });
+  const filter = createFilter(
+    include || [`*.${defaultTsExtensionsGlob}`, `**/*.${defaultTsExtensionsGlob}`],
+    exclude,
+    {
+      resolve: filterRoot ?? parsedOptions.options.rootDir
+    }
+  );
   parsedOptions.fileNames = parsedOptions.fileNames.filter(filter);
 
   const formatHost = createFormattingHost(ts, parsedOptions.options);
@@ -110,7 +119,7 @@ export default function typescript(options: RollupTypescriptOptions = {}): Plugi
       const resolved = resolveModule(importee, containingFile);
 
       if (resolved) {
-        if (resolved.extension === '.d.ts') return null;
+        if (isTypeDeclarationFile(resolved)) return null;
         return path.normalize(resolved.resolvedFileName);
       }
 
@@ -173,3 +182,16 @@ export default function typescript(options: RollupTypescriptOptions = {}): Plugi
     }
   };
 }
+
+/**
+ * Check if the given resolved TypeScript module is a type declaration.
+ */
+function isTypeDeclarationFile(resolved: ResolvedModuleFull) {
+  const lastDotDDot = resolved.extension.lastIndexOf('.d.');
+  if (lastDotDDot < 0) return false;
+
+  const lastDot = resolved.extension.lastIndexOf('.');
+
+  // Ensure the dot is the second one in `.d.`
+  return lastDot === lastDotDDot + 2;
+}

packages/typescript/test/fixtures/cts/main.mts

@@ -0,0 +1,3 @@
+const answer = 42;
+// eslint-disable-next-line no-console
+console.log(`the answer is ${answer}`);

packages/typescript/test/fixtures/cts/tsconfig.json

@@ -0,0 +1,6 @@
+{
+  "compilerOptions": {
+    "module": "Node16",
+    "moduleResolution": "Node16"
+  }
+}

packages/typescript/test/fixtures/dcts/main.cts

@@ -0,0 +1,5 @@
+/* eslint-disable */
+// @ts-ignore
+import { foo } from 'an-import';
+
+foo();

packages/typescript/test/fixtures/dcts/tsconfig.json

@@ -0,0 +1,6 @@
+{
+  "compilerOptions": {
+    "module": "Node16",
+    "moduleResolution": "Node16"
+  }
+}

packages/typescript/test/fixtures/dmts/main.mts

@@ -0,0 +1,5 @@
+/* eslint-disable */
+// @ts-ignore
+import { foo } from 'an-import';
+
+foo();

packages/typescript/test/fixtures/dmts/tsconfig.json

@@ -0,0 +1,6 @@
+{
+  "compilerOptions": {
+    "module": "Node16",
+    "moduleResolution": "Node16"
+  }
+}

packages/typescript/test/fixtures/mts/main.mts

@@ -0,0 +1,3 @@
+const answer = 42;
+// eslint-disable-next-line no-console
+console.log(`the answer is ${answer}`);

packages/typescript/test/fixtures/mts/tsconfig.json

@@ -0,0 +1,6 @@
+{
+  "compilerOptions": {
+    "module": "Node16",
+    "moduleResolution": "Node16"
+  }
+}

packages/typescript/test/test.js

@@ -16,7 +16,7 @@ test.beforeEach(() => process.chdir(__dirname));
 
 const outputOptions = { format: 'esm' };
 
-test.serial('runs code through typescript', async (t) => {
+test.serial('runs ts code through typescript', async (t) => {
   const bundle = await rollup({
     input: 'fixtures/basic/main.ts',
     plugins: [typescript({ tsconfig: 'fixtures/basic/tsconfig.json', target: 'es5' })],
@@ -28,6 +28,30 @@ test.serial('runs code through typescript', async (t) => {
   t.false(code.includes('const'), code);
 });
 
+test.serial('runs mts code through typescript', async (t) => {
+  const bundle = await rollup({
+    input: 'fixtures/mts/main.ts',
+    plugins: [typescript({ tsconfig: 'fixtures/mts/tsconfig.json', target: 'es5' })],
+    onwarn
+  });
+  const code = await getCode(bundle, outputOptions);
+
+  t.false(code.includes('number'), code);
+  t.false(code.includes('const'), code);
+});
+
+test.serial('runs cts code through typescript', async (t) => {
+  const bundle = await rollup({
+    input: 'fixtures/cts/main.ts',
+    plugins: [typescript({ tsconfig: 'fixtures/cts/tsconfig.json', target: 'es5' })],
+    onwarn
+  });
+  const code = await getCode(bundle, outputOptions);
+
+  t.false(code.includes('number'), code);
+  t.false(code.includes('const'), code);
+});
+
 test.serial('runs code through typescript with compilerOptions', async (t) => {
   const bundle = await rollup({
     input: 'fixtures/basic/main.ts',
@@ -375,6 +399,28 @@ test.serial('should not resolve .d.ts files', async (t) => {
   t.deepEqual(imports, ['an-import']);
 });
 
+test.serial('should not resolve .d.cts files', async (t) => {
+  const bundle = await rollup({
+    input: 'fixtures/dts/main.cts',
+    plugins: [typescript({ tsconfig: 'fixtures/dcts/tsconfig.json' })],
+    onwarn,
+    external: ['an-import']
+  });
+  const imports = bundle.cache.modules[0].dependencies;
+  t.deepEqual(imports, ['an-import']);
+});
+
+test.serial('should not resolve .d.mts files', async (t) => {
+  const bundle = await rollup({
+    input: 'fixtures/dts/main.mts',
+    plugins: [typescript({ tsconfig: 'fixtures/dmts/tsconfig.json' })],
+    onwarn,
+    external: ['an-import']
+  });
+  const imports = bundle.cache.modules[0].dependencies;
+  t.deepEqual(imports, ['an-import']);
+});
+
 test.serial('should transpile JSX if enabled', async (t) => {
   process.chdir('fixtures/jsx');