util: set super_ property to non-enumerable

Using `util.inherits()` adds a `super_` property to the constructor
and inspecting that constructor is going to show that property even
though it's not useful for the user.

Therefore the property is now set as non-enumerable and such entries
are not visible by default when using `console.log()` or
`util.inspect()`.

PR-URL: https://github.com/nodejs/node/pull/23107
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/util.js

@@ -285,7 +285,11 @@ function inherits(ctor, superCtor) {
     throw new ERR_INVALID_ARG_TYPE('superCtor.prototype',
                                    'Function', superCtor.prototype);
   }
-  ctor.super_ = superCtor;
+  Object.defineProperty(ctor, 'super_', {
+    value: superCtor,
+    writable: true,
+    configurable: true
+  });
   Object.setPrototypeOf(ctor.prototype, superCtor.prototype);
 }
 

test/parallel/test-util-inherits.js

@@ -18,7 +18,15 @@ function B(value) {
 inherits(B, A);
 B.prototype.b = function() { return this._b; };
 
-assert.strictEqual(B.super_, A);
+assert.deepStrictEqual(
+  Object.getOwnPropertyDescriptor(B, 'super_'),
+  {
+    value: A,
+    enumerable: false,
+    configurable: true,
+    writable: true
+  }
+);
 
 const b = new B('b');
 assert.strictEqual(b.a(), 'a');