From 90473733a525862b4aba8260e7008cb081b82d2d Mon Sep 17 00:00:00 2001
From: Michael Jackson <michael@jackson.us>
Date: Tue, 2 Jul 2019 21:16:28 -0700
Subject: [PATCH] Do not return non-ESM on ?module requests

---
 modules/middleware/fetchPackage.js | 26 ++++++++++++++++++++++----
 1 file changed, 22 insertions(+), 4 deletions(-)

diff --git a/modules/middleware/fetchPackage.js b/modules/middleware/fetchPackage.js
index baefecf..6878d72 100644
--- a/modules/middleware/fetchPackage.js
+++ b/modules/middleware/fetchPackage.js
@@ -47,10 +47,28 @@ function filenameRedirect(req, res) {
   let filename;
   if (req.query.module != null) {
     // See https://github.com/rollup/rollup/wiki/pkg.module
-    filename =
-      req.packageConfig.module ||
-      req.packageConfig['jsnext:main'] ||
-      '/index.js';
+    filename = req.packageConfig.module || req.packageConfig['jsnext:main'];
+
+    if (!filename) {
+      // https://nodejs.org/api/esm.html#esm_code_package_json_code_code_type_code_field
+      if (req.packageConfig.type === 'module') {
+        // Use whatever is in pkg.main or index.js
+        filename = req.packageConfig.main || '/index.js';
+      } else if (
+        req.packageConfig.main &&
+        /\.mjs$/.test(req.packageConfig.main)
+      ) {
+        // Use .mjs file in pkg.main
+        filename = req.packageConfig.main;
+      }
+    }
+
+    if (!filename) {
+      return res
+        .status(404)
+        .type('text')
+        .send(`Package ${req.packageSpec} does not contain an ES module`);
+    }
   } else if (
     req.query.main &&
     req.packageConfig[req.query.main] &&