diff --git a/src/loaders/js.ts b/src/loaders/js.ts
index e9e1ff3..c3bf89b 100644
--- a/src/loaders/js.ts
+++ b/src/loaders/js.ts
@@ -6,11 +6,13 @@ import type { Loader, LoaderResult } from "../loader";
 const DECLARATION_RE = /\.d\.[cm]?ts$/;
 const CM_LETTER_RE = /(?<=\.)(c|m)(?=[jt]s$)/;
 
+const KNOWN_EXT_RE = /\.(c|m)?[jt]sx?$/;
+
+const TS_EXTS = new Set([".ts", ".mts", ".cts"]);
+
 export const jsLoader: Loader = async (input, { options }) => {
-  if (
-    ![".ts", ".js", ".cjs", ".mjs", ".tsx", ".jsx"].includes(input.extension) ||
-    DECLARATION_RE.test(input.path)
-  ) {
+  if (!KNOWN_EXT_RE.test(input.path) || DECLARATION_RE.test(input.path)) {
+    console.log("Ignoring file with known extension:", input.path);
     return;
   }
 
@@ -32,7 +34,7 @@ export const jsLoader: Loader = async (input, { options }) => {
   }
 
   // typescript => js
-  if (input.extension === ".ts") {
+  if (TS_EXTS.has(input.extension)) {
     contents = await transform(contents, {
       ...options.esbuild,
       loader: "ts",
diff --git a/test/fixture/src/ts/test1.mts b/test/fixture/src/ts/test1.mts
new file mode 100644
index 0000000..869a185
--- /dev/null
+++ b/test/fixture/src/ts/test1.mts
@@ -0,0 +1 @@
+export default "test1.mts" as string;
diff --git a/test/fixture/src/ts/test2.cts b/test/fixture/src/ts/test2.cts
new file mode 100644
index 0000000..1d61271
--- /dev/null
+++ b/test/fixture/src/ts/test2.cts
@@ -0,0 +1 @@
+module.exports = "test2.cts" as string;
diff --git a/test/index.test.ts b/test/index.test.ts
index 337560d..e848d1d 100644
--- a/test/index.test.ts
+++ b/test/index.test.ts
@@ -26,6 +26,8 @@ describe("mkdist", () => {
         "dist/components/tsx.mjs",
         "dist/bar/index.mjs",
         "dist/bar/esm.mjs",
+        "dist/ts/test1.mjs",
+        "dist/ts/test2.mjs",
       ]
         .map((f) => resolve(rootDir, f))
         .sort()
@@ -105,6 +107,10 @@ describe("mkdist", () => {
         "dist/bar/index.d.ts",
         "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",
       ]
         .map((f) => resolve(rootDir, f))
         .sort()
@@ -180,6 +186,8 @@ describe("mkdist", () => {
         "dist/components/tsx.mjs",
         "dist/bar/index.mjs",
         "dist/bar/esm.mjs",
+        "dist/ts/test1.mjs",
+        "dist/ts/test2.mjs",
       ]
         .map((f) => resolve(rootDir, f))
         .sort()