module: make module.parent truthy when loaded from ESM

doc/api/modules.md

@@ -912,16 +912,23 @@ added: v0.1.16
 deprecated:
   - v14.6.0
   - v12.19.0
+changes:
+  - version: REPLACEME
+    pr-url: https://github.com/nodejs/node/pull/00000
+    description: When the module is loaded by something that is not a CommonJS
+                 module, the value of `module.parent` is truthy. On previous
+                 versions, it is `undefined`.
 -->
 
 > Stability: 0 - Deprecated: Please use [`require.main`][] and
 > [`module.children`][] instead.
 
-* {module | null | undefined}
+* {module | null | Object}
 
 The module that first required this one, or `null` if the current module is the
-entry point of the current process, or `undefined` if the module was loaded by
-something that is not a CommonJS module (E.G.: REPL or `import`).
+entry point of the current process, or an instance of an add-hoc `UnknownModule`
+class if the module was loaded by something that is not a CommonJS module (E.G.:
+REPL or `import`).
 
 ### `module.path`
 <!-- YAML

lib/internal/modules/cjs/loader.js

@@ -241,7 +241,13 @@ ObjectDefineProperty(Module.prototype, 'isPreloading', isPreloadingDesc);
 ObjectDefineProperty(NativeModule.prototype, 'isPreloading', isPreloadingDesc);
 
 function getModuleParent() {
-  return moduleParentCache.get(this);
+  const parent = moduleParentCache.get(this);
+  if (parent === undefined) {
+    const parent = new (class UnknownModule {})();
+    moduleParentCache.set(this, parent);
+    return parent;
+  }
+  return parent;
 }
 
 function setModuleParent(value) {

test/fixtures/cjs-module-parent.js

@@ -0,0 +1,5 @@
+'use strict';
+const assert = require('assert');
+
+// Accessing module.parent should not be falsy.
+assert.ok(module.parent)

test/parallel/test-module-parent-deprecation-no-warning-for-cjs-users.js

@@ -0,0 +1,7 @@
+'use strict';
+
+require('../common');
+const fixtures = require('../common/fixtures');
+
+// Accessing module.parent from a child module should not raise a warning.
+require(fixtures.path('cjs-module-parent.js'));

test/parallel/test-module-parent-deprecation-warning-for-esm-users.mjs

@@ -0,0 +1,7 @@
+import { mustCall } from '../common/index.mjs';
+import fixtures from '../common/fixtures.js';
+
+// Accessing module.parent from a child module should raise a deprecation
+// warning when imported from ESM.
+import(fixtures.path('cjs-module-parent.js'))
+    .then(mustCall());