module: fix unintended mutation

Refs: #46061 (comment)
PR-URL: #46108
Reviewed-By: Rafael Gonzaga <rafael.nunu@hotmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Geoffrey Booth <webadmin@geoffreybooth.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Jacob Smith <jacob@frende.me>

lib/internal/modules/cjs/loader.js

@@ -29,6 +29,7 @@ const {
   ArrayPrototypeJoin,
   ArrayPrototypeMap,
   ArrayPrototypePush,
+  ArrayPrototypePushApply,
   ArrayPrototypeSlice,
   ArrayPrototypeSplice,
   ArrayPrototypeUnshift,
@@ -668,12 +669,11 @@ Module._findPath = function(request, paths, isMain) {
       return filename;
     }
 
-    if (exts === undefined) {
-      exts = [''];
-    } else {
-      ArrayPrototypeUnshift(exts, '');
+    const extensions = [''];
+    if (exts !== undefined) {
+      ArrayPrototypePushApply(extensions, exts);
     }
-    reportModuleNotFoundToWatchMode(basePath, exts);
+    reportModuleNotFoundToWatchMode(basePath, extensions);
   }
 
   return false;

test/fixtures/require-resolve.js

@@ -98,3 +98,8 @@ assert.strictEqual(
   require.resolve('./printA.js', {}),
   require.resolve('./printA.js')
 );
+
+assert.strictEqual(
+  require.resolve('no_index/'),
+  path.join(__dirname, 'node_modules', 'no_index', 'lib', 'index.js'),
+)