diff --git a/src/loaders/js.ts b/src/loaders/js.ts
index e7e79fe..918f13b 100644
--- a/src/loaders/js.ts
+++ b/src/loaders/js.ts
@@ -19,15 +19,20 @@ export const jsLoader: Loader = async (input, { options }) => {
 
   let contents = await input.getContents();
 
+  const isCjs = options.format === "cjs";
+  let extension = isCjs ? ".js" : ".mjs"; // TODO: Default to .cjs in next major version
+  if (options.ext) {
+    extension = options.ext.startsWith(".") ? options.ext : `.${options.ext}`;
+  }
+
   // declaration
   if (options.declaration && !input.srcPath?.match(DECLARATION_RE)) {
-    const cm = input.srcPath?.match(CM_LETTER_RE)?.[0] || "";
-    const extension = `.d.${cm}ts`;
+    const cm = extension.match(CM_LETTER_RE)?.[0] || "";
     output.push({
       contents,
       srcPath: input.srcPath,
       path: input.path,
-      extension,
+      extension: `.d.${cm}ts`,
       declaration: true,
     });
   }
@@ -46,7 +51,6 @@ export const jsLoader: Loader = async (input, { options }) => {
   }
 
   // esm => cjs
-  const isCjs = options.format === "cjs";
   if (isCjs) {
     contents = jiti("")
       .transform({ source: contents, retainLines: false })
@@ -55,11 +59,6 @@ export const jsLoader: Loader = async (input, { options }) => {
       .replace("module.exports = void 0;", "");
   }
 
-  let extension = isCjs ? ".js" : ".mjs"; // TODO: Default to .cjs in next major version
-  if (options.ext) {
-    extension = options.ext.startsWith(".") ? options.ext : `.${options.ext}`;
-  }
-
   output.push({
     contents,
     path: input.path,
diff --git a/test/index.test.ts b/test/index.test.ts
index 854fc2e..455ee02 100644
--- a/test/index.test.ts
+++ b/test/index.test.ts
@@ -106,47 +106,48 @@ describe("mkdist", () => {
     expect(writtenFiles.sort()).toEqual(
       [
         "dist/README.md",
-        "dist/bar.d.ts",
+        "dist/bar.d.mts",
         "dist/bar.mjs",
         "dist/demo.css",
-        "dist/dir-export.d.ts",
+        "dist/dir-export.d.mts",
         "dist/dir-export.mjs",
         "dist/foo.mjs",
         "dist/foo.d.ts",
+        "dist/foo.d.mts",
         "dist/index.mjs",
-        "dist/index.d.ts",
+        "dist/index.d.mts",
         "dist/star/index.mjs",
-        "dist/star/index.d.ts",
+        "dist/star/index.d.mts",
         "dist/star/other.mjs",
-        "dist/star/other.d.ts",
+        "dist/star/other.d.mts",
         "dist/types.d.ts",
         "dist/components/index.mjs",
-        "dist/components/index.d.ts",
+        "dist/components/index.d.mts",
         "dist/components/blank.vue",
-        "dist/components/blank.vue.d.ts",
+        "dist/components/blank.vue.d.mts",
         "dist/components/js.vue",
-        "dist/components/js.vue.d.ts",
+        "dist/components/js.vue.d.mts",
         "dist/components/script-multi-block.vue",
-        "dist/components/script-multi-block.vue.d.ts",
+        "dist/components/script-multi-block.vue.d.mts",
         "dist/components/script-setup-ts.vue",
-        "dist/components/script-setup-ts.vue.d.ts",
+        "dist/components/script-setup-ts.vue.d.mts",
         "dist/components/ts.vue",
-        "dist/components/ts.vue.d.ts",
+        "dist/components/ts.vue.d.mts",
         "dist/components/jsx.mjs",
         "dist/components/tsx.mjs",
-        "dist/components/jsx.d.ts",
-        "dist/components/tsx.d.ts",
+        "dist/components/jsx.d.mts",
+        "dist/components/tsx.d.mts",
         "dist/bar/index.mjs",
-        "dist/bar/index.d.ts",
+        "dist/bar/index.d.mts",
         "dist/bar/esm.mjs",
         "dist/bar/esm.d.mts",
         "dist/ts/test1.mjs",
         "dist/ts/test2.mjs",
         "dist/ts/test1.d.mts",
-        "dist/ts/test2.d.cts",
+        "dist/ts/test2.d.mts",
         "dist/nested.css",
         "dist/prop-types/index.mjs",
-        "dist/prop-types/index.d.ts",
+        "dist/prop-types/index.d.mts",
       ]
         .map((f) => resolve(rootDir, f))
         .sort(),
@@ -156,14 +157,14 @@ describe("mkdist", () => {
       "manual declaration",
     );
 
-    expect(await readFile(resolve(rootDir, "dist/star/index.d.ts"), "utf8"))
+    expect(await readFile(resolve(rootDir, "dist/star/index.d.mts"), "utf8"))
       .toMatchInlineSnapshot(`
         "export * from "./other.js";
         export type { Other } from "./other.js";
         "
       `);
 
-    expect(await readFile(resolve(rootDir, "dist/dir-export.d.ts"), "utf8"))
+    expect(await readFile(resolve(rootDir, "dist/dir-export.d.mts"), "utf8"))
       .toMatchInlineSnapshot(`
         "export { default as bar } from "./bar.js";
         export * from "./star/index.js";
@@ -175,7 +176,7 @@ describe("mkdist", () => {
     ).toMatch("declare");
 
     expect(
-      await readFile(resolve(rootDir, "dist/components/index.d.ts"), "utf8"),
+      await readFile(resolve(rootDir, "dist/components/index.d.mts"), "utf8"),
     ).toMatchInlineSnapshot(`
       "export * as jsx from "./jsx.jsx.js";
       export * as tsx from "./tsx.tsx.js";
@@ -187,7 +188,7 @@ describe("mkdist", () => {
     `);
 
     expect(
-      await readFile(resolve(rootDir, "dist/components/ts.vue.d.ts"), "utf8"),
+      await readFile(resolve(rootDir, "dist/components/ts.vue.d.mts"), "utf8"),
     ).toMatchInlineSnapshot(`
       "declare const _default: import("vue").DefineComponent<{}, {}, {
           test: string;
@@ -199,7 +200,7 @@ describe("mkdist", () => {
 
     expect(
       await readFile(
-        resolve(rootDir, "dist/components/blank.vue.d.ts"),
+        resolve(rootDir, "dist/components/blank.vue.d.mts"),
         "utf8",
       ),
     ).toMatchInlineSnapshot(`
@@ -210,7 +211,7 @@ describe("mkdist", () => {
 
     expect(
       await readFile(
-        resolve(rootDir, "dist/components/script-multi-block.vue.d.ts"),
+        resolve(rootDir, "dist/components/script-multi-block.vue.d.mts"),
         "utf8",
       ),
     ).toMatchInlineSnapshot(`
@@ -233,7 +234,7 @@ describe("mkdist", () => {
 
     expect(
       await readFile(
-        resolve(rootDir, "dist/components/script-setup-ts.vue.d.ts"),
+        resolve(rootDir, "dist/components/script-setup-ts.vue.d.mts"),
         "utf8",
       ),
     ).toMatchInlineSnapshot(`
@@ -537,47 +538,48 @@ describe("mkdist with vue-tsc v1", () => {
     expect(writtenFiles.sort()).toEqual(
       [
         "dist/README.md",
-        "dist/bar.d.ts",
+        "dist/bar.d.mts",
         "dist/bar.mjs",
         "dist/demo.css",
-        "dist/dir-export.d.ts",
+        "dist/dir-export.d.mts",
         "dist/dir-export.mjs",
         "dist/foo.mjs",
         "dist/foo.d.ts",
+        "dist/foo.d.mts",
         "dist/index.mjs",
-        "dist/index.d.ts",
+        "dist/index.d.mts",
         "dist/star/index.mjs",
-        "dist/star/index.d.ts",
+        "dist/star/index.d.mts",
         "dist/star/other.mjs",
-        "dist/star/other.d.ts",
+        "dist/star/other.d.mts",
         "dist/types.d.ts",
         "dist/components/index.mjs",
-        "dist/components/index.d.ts",
+        "dist/components/index.d.mts",
         "dist/components/blank.vue",
-        "dist/components/blank.vue.d.ts",
+        "dist/components/blank.vue.d.mts",
         "dist/components/js.vue",
-        "dist/components/js.vue.d.ts",
+        "dist/components/js.vue.d.mts",
         "dist/components/script-multi-block.vue",
-        "dist/components/script-multi-block.vue.d.ts",
+        "dist/components/script-multi-block.vue.d.mts",
         "dist/components/script-setup-ts.vue",
-        "dist/components/script-setup-ts.vue.d.ts",
+        "dist/components/script-setup-ts.vue.d.mts",
         "dist/components/ts.vue",
-        "dist/components/ts.vue.d.ts",
+        "dist/components/ts.vue.d.mts",
         "dist/components/jsx.mjs",
         "dist/components/tsx.mjs",
-        "dist/components/jsx.d.ts",
-        "dist/components/tsx.d.ts",
+        "dist/components/jsx.d.mts",
+        "dist/components/tsx.d.mts",
         "dist/bar/index.mjs",
-        "dist/bar/index.d.ts",
+        "dist/bar/index.d.mts",
         "dist/bar/esm.mjs",
         "dist/bar/esm.d.mts",
         "dist/ts/test1.mjs",
         "dist/ts/test2.mjs",
         "dist/ts/test1.d.mts",
-        "dist/ts/test2.d.cts",
+        "dist/ts/test2.d.mts",
         "dist/nested.css",
         "dist/prop-types/index.mjs",
-        "dist/prop-types/index.d.ts",
+        "dist/prop-types/index.d.mts",
       ]
         .map((f) => resolve(rootDir, f))
         .sort(),
@@ -587,7 +589,7 @@ describe("mkdist with vue-tsc v1", () => {
       "manual declaration",
     );
 
-    expect(await readFile(resolve(rootDir, "dist/star/index.d.ts"), "utf8"))
+    expect(await readFile(resolve(rootDir, "dist/star/index.d.mts"), "utf8"))
       .toMatchInlineSnapshot(`
         "export * from "./other.js";
         export type { Other } from "./other.js";
@@ -598,7 +600,7 @@ describe("mkdist with vue-tsc v1", () => {
     ).toMatch("declare");
 
     expect(
-      await readFile(resolve(rootDir, "dist/components/index.d.ts"), "utf8"),
+      await readFile(resolve(rootDir, "dist/components/index.d.mts"), "utf8"),
     ).toMatchInlineSnapshot(`
       "export * as jsx from "./jsx.jsx.js";
       export * as tsx from "./tsx.tsx.js";
@@ -611,7 +613,7 @@ describe("mkdist with vue-tsc v1", () => {
 
     expect(
       await readFile(
-        resolve(rootDir, "dist/components/blank.vue.d.ts"),
+        resolve(rootDir, "dist/components/blank.vue.d.mts"),
         "utf8",
       ),
     ).toMatchInlineSnapshot(`
@@ -621,7 +623,7 @@ describe("mkdist with vue-tsc v1", () => {
     `);
 
     expect(
-      await readFile(resolve(rootDir, "dist/components/ts.vue.d.ts"), "utf8"),
+      await readFile(resolve(rootDir, "dist/components/ts.vue.d.mts"), "utf8"),
     ).toMatchInlineSnapshot(`
       "declare const _default: import("vue").DefineComponent<{}, {}, {
           test: string;
@@ -693,7 +695,7 @@ describe("mkdist with vue-tsc v1", () => {
 
     expect(
       await readFile(
-        resolve(rootDir, "dist/components/script-multi-block.vue.d.ts"),
+        resolve(rootDir, "dist/components/script-multi-block.vue.d.mts"),
         "utf8",
       ),
     ).toMatchInlineSnapshot(`
@@ -716,7 +718,7 @@ describe("mkdist with vue-tsc v1", () => {
 
     expect(
       await readFile(
-        resolve(rootDir, "dist/components/script-setup-ts.vue.d.ts"),
+        resolve(rootDir, "dist/components/script-setup-ts.vue.d.mts"),
         "utf8",
       ),
     ).toMatchInlineSnapshot(`
@@ -794,47 +796,48 @@ describe("mkdist with vue-tsc ~v2.0.21", () => {
     expect(writtenFiles.sort()).toEqual(
       [
         "dist/README.md",
-        "dist/bar.d.ts",
+        "dist/bar.d.mts",
         "dist/bar.mjs",
         "dist/demo.css",
-        "dist/dir-export.d.ts",
+        "dist/dir-export.d.mts",
         "dist/dir-export.mjs",
         "dist/foo.mjs",
+        "dist/foo.d.mts",
         "dist/foo.d.ts",
         "dist/index.mjs",
-        "dist/index.d.ts",
+        "dist/index.d.mts",
         "dist/star/index.mjs",
-        "dist/star/index.d.ts",
+        "dist/star/index.d.mts",
         "dist/star/other.mjs",
-        "dist/star/other.d.ts",
+        "dist/star/other.d.mts",
         "dist/types.d.ts",
         "dist/components/index.mjs",
-        "dist/components/index.d.ts",
+        "dist/components/index.d.mts",
         "dist/components/blank.vue",
-        "dist/components/blank.vue.d.ts",
+        "dist/components/blank.vue.d.mts",
         "dist/components/js.vue",
-        "dist/components/js.vue.d.ts",
+        "dist/components/js.vue.d.mts",
         "dist/components/script-multi-block.vue",
-        "dist/components/script-multi-block.vue.d.ts",
+        "dist/components/script-multi-block.vue.d.mts",
         "dist/components/script-setup-ts.vue",
-        "dist/components/script-setup-ts.vue.d.ts",
+        "dist/components/script-setup-ts.vue.d.mts",
         "dist/components/ts.vue",
-        "dist/components/ts.vue.d.ts",
+        "dist/components/ts.vue.d.mts",
         "dist/components/jsx.mjs",
         "dist/components/tsx.mjs",
-        "dist/components/jsx.d.ts",
-        "dist/components/tsx.d.ts",
+        "dist/components/jsx.d.mts",
+        "dist/components/tsx.d.mts",
         "dist/bar/index.mjs",
-        "dist/bar/index.d.ts",
+        "dist/bar/index.d.mts",
         "dist/bar/esm.mjs",
         "dist/bar/esm.d.mts",
         "dist/ts/test1.mjs",
         "dist/ts/test2.mjs",
         "dist/ts/test1.d.mts",
-        "dist/ts/test2.d.cts",
+        "dist/ts/test2.d.mts",
         "dist/nested.css",
         "dist/prop-types/index.mjs",
-        "dist/prop-types/index.d.ts",
+        "dist/prop-types/index.d.mts",
       ]
         .map((f) => resolve(rootDir, f))
         .sort(),
@@ -844,7 +847,7 @@ describe("mkdist with vue-tsc ~v2.0.21", () => {
       "manual declaration",
     );
 
-    expect(await readFile(resolve(rootDir, "dist/star/index.d.ts"), "utf8"))
+    expect(await readFile(resolve(rootDir, "dist/star/index.d.mts"), "utf8"))
       .toMatchInlineSnapshot(`
         "export * from "./other.js";
         export type { Other } from "./other.js";
@@ -855,7 +858,7 @@ describe("mkdist with vue-tsc ~v2.0.21", () => {
     ).toMatch("declare");
 
     expect(
-      await readFile(resolve(rootDir, "dist/components/index.d.ts"), "utf8"),
+      await readFile(resolve(rootDir, "dist/components/index.d.mts"), "utf8"),
     ).toMatchInlineSnapshot(`
       "export * as jsx from "./jsx.jsx.js";
       export * as tsx from "./tsx.tsx.js";
@@ -868,7 +871,7 @@ describe("mkdist with vue-tsc ~v2.0.21", () => {
 
     expect(
       await readFile(
-        resolve(rootDir, "dist/components/blank.vue.d.ts"),
+        resolve(rootDir, "dist/components/blank.vue.d.mts"),
         "utf8",
       ),
     ).toMatchInlineSnapshot(`
@@ -878,7 +881,7 @@ describe("mkdist with vue-tsc ~v2.0.21", () => {
     `);
 
     expect(
-      await readFile(resolve(rootDir, "dist/components/ts.vue.d.ts"), "utf8"),
+      await readFile(resolve(rootDir, "dist/components/ts.vue.d.mts"), "utf8"),
     ).toMatchInlineSnapshot(`
       "declare const _default: import("vue").DefineComponent<{}, {}, {
           test: string;
@@ -918,7 +921,7 @@ describe("mkdist with vue-tsc ~v2.0.21", () => {
 
     expect(
       await readFile(
-        resolve(rootDir, "dist/components/script-multi-block.vue.d.ts"),
+        resolve(rootDir, "dist/components/script-multi-block.vue.d.mts"),
         "utf8",
       ),
     ).toMatchInlineSnapshot(`
@@ -941,7 +944,7 @@ describe("mkdist with vue-tsc ~v2.0.21", () => {
 
     expect(
       await readFile(
-        resolve(rootDir, "dist/components/script-setup-ts.vue.d.ts"),
+        resolve(rootDir, "dist/components/script-setup-ts.vue.d.mts"),
         "utf8",
       ),
     ).toMatchInlineSnapshot(`