diff --git a/server/middleware/findFile.js b/server/middleware/findFile.js
index 9221eba..81a3ee7 100644
--- a/server/middleware/findFile.js
+++ b/server/middleware/findFile.js
@@ -85,7 +85,11 @@ function searchEntries(tarballStream, entryName, wantsHTML) {
         // and the client wants HTML.
         if (
           entry.name === entryName ||
-          (wantsHTML && entry.name === entryName + "/index.html")
+          (wantsHTML && entry.name === entryName + "/index.html") ||
+          // Allow accessing e.g. `/index.js` or `/index.json` using
+          // `/index` for compatibility with CommonJS
+          (!wantsHTML && entry.name === entryName + ".js") ||
+          (!wantsHTML && entry.name === entryName + ".json")
         ) {
           foundEntry = entry;
         }