util: fix module inspection & instanceof check during inspect

lib/internal/util/inspect.js

@@ -534,6 +534,14 @@ function getEmptyFormatArray() {
   return [];
 }
 
+function isInstanceof(object, proto) {
+  try {
+    return object instanceof proto;
+  } catch {
+    return false;
+  }
+}
+
 function getConstructorName(obj, ctx, recurseTimes, protoProps) {
   let firstProto;
   const tmp = obj;
@@ -542,7 +550,7 @@ function getConstructorName(obj, ctx, recurseTimes, protoProps) {
     if (descriptor !== undefined &&
         typeof descriptor.value === 'function' &&
         descriptor.value.name !== '' &&
-        tmp instanceof descriptor.value) {
+        isInstanceof(tmp, descriptor.value)) {
       if (protoProps !== undefined &&
          (firstProto !== obj ||
          !builtInObjects.has(descriptor.value.name))) {
@@ -644,7 +652,7 @@ function addPrototypeProperties(ctx, main, obj, recurseTimes, output) {
 
 function getPrefix(constructor, tag, fallback, size = '') {
   if (constructor === null) {
-    if (tag !== '') {
+    if (tag !== '' && fallback !== tag) {
       return `[${fallback}${size}: null prototype] [${tag}] `;
     }
     return `[${fallback}${size}: null prototype] `;
@@ -978,7 +986,7 @@ function formatRaw(ctx, value, recurseTimes, typedArray) {
       braces[0] = `${getPrefix(constructor, tag, 'WeakMap')}{`;
       formatter = ctx.showHidden ? formatWeakMap : formatWeakCollection;
     } else if (isModuleNamespaceObject(value)) {
-      braces[0] = `[${tag}] {`;
+      braces[0] = `${getPrefix(constructor, tag, 'Module')}{`;
       // Special handle keys for namespace objects.
       formatter = formatNamespaceObject.bind(null, keys);
     } else if (isBoxedPrimitive(value)) {

test/es-module/test-esm-loader-custom-condition.mjs

@@ -1,7 +1,21 @@
 // Flags: --experimental-loader ./test/fixtures/es-module-loaders/loader-with-custom-condition.mjs
 import '../common/index.mjs';
 import assert from 'assert';
+import util from 'util';
 
 import * as ns from '../fixtures/es-modules/conditional-exports.mjs';
 
 assert.deepStrictEqual({ ...ns }, { default: 'from custom condition' });
+
+assert.strictEqual(
+  util.inspect(ns, { showHidden: false }),
+  "[Module: null prototype] { default: 'from custom condition' }"
+);
+
+assert.strictEqual(
+  util.inspect(ns, { showHidden: true }),
+  '[Module: null prototype] {\n' +
+  "  default: 'from custom condition',\n" +
+  "  [Symbol(Symbol.toStringTag)]: 'Module'\n" +
+  '}'
+);

test/parallel/test-repl-import-referrer.js

@@ -16,7 +16,10 @@ child.stdout.on('data', (data) => {
 
 child.on('exit', common.mustCall(() => {
   const results = output.replace(/^> /mg, '').split('\n').slice(2);
-  assert.deepStrictEqual(results, ['[Module] { message: \'A message\' }', '']);
+  assert.deepStrictEqual(
+    results,
+    ['[Module: null prototype] { message: \'A message\' }', '']
+  );
 }));
 
 child.stdin.write('await import(\'./message.mjs\');\n');

test/parallel/test-util-inspect-namespace.js

@@ -11,7 +11,10 @@ const { inspect } = require('util');
   await m.link(() => 0);
   assert.strictEqual(
     inspect(m.namespace),
-    '[Module] { a: <uninitialized>, b: undefined }');
+    '[Module: null prototype] { a: <uninitialized>, b: undefined }');
   await m.evaluate();
-  assert.strictEqual(inspect(m.namespace), '[Module] { a: 1, b: 2 }');
+  assert.strictEqual(
+    inspect(m.namespace),
+    '[Module: null prototype] { a: 1, b: 2 }'
+  );
 })().then(common.mustCall());

test/parallel/test-util-inspect.js

@@ -2866,6 +2866,17 @@ assert.strictEqual(
   );
 }
 
+// Check that prototypes with a null prototype are inspectable.
+// Regression test for https://github.com/nodejs/node/issues/35730
+{
+  function Func() {}
+  Func.prototype = null;
+  const object = {};
+  object.constructor = Func;
+
+  assert.strictEqual(util.inspect(object), '{ constructor: [Function: Func] }');
+}
+
 // Test changing util.inspect.colors colors and aliases.
 {
   const colors = util.inspect.colors;