diff --git a/src/lib/functions/netlify-function.ts b/src/lib/functions/netlify-function.ts
index 8acd316709c..f4f55501771 100644
--- a/src/lib/functions/netlify-function.ts
+++ b/src/lib/functions/netlify-function.ts
@@ -121,7 +121,7 @@ export default class NetlifyFunction {
     }
 
     if (extension === '.js') {
-      return '.js'
+      return '.mjs'
     }
   }
 
diff --git a/src/lib/functions/runtimes/js/builders/zisi.ts b/src/lib/functions/runtimes/js/builders/zisi.ts
index e85de6f0429..1d08ab35112 100644
--- a/src/lib/functions/runtimes/js/builders/zisi.ts
+++ b/src/lib/functions/runtimes/js/builders/zisi.ts
@@ -175,7 +175,7 @@ export default async function handler({ config, directory, errorExit, func, meta
     featureFlags.zisi_pure_esm_mjs = true
   } else {
     // We must use esbuild for certain file extensions.
-    const mustTranspile = ['.js', '.ts', '.mts', '.cts'].includes(path.extname(func.mainFile))
+    const mustTranspile = ['.mjs', '.ts', '.mts', '.cts'].includes(path.extname(func.mainFile))
     const mustUseEsbuild = hasTypeModule || mustTranspile
 
     if (mustUseEsbuild && !functionsConfig['*'].nodeBundler) {