From 014a9fd46fb312816422bb56326bae81187abf15 Mon Sep 17 00:00:00 2001
From: Myles Borins <mylesborins@google.com>
Date: Thu, 25 Apr 2019 17:32:04 -0400
Subject: [PATCH] module: throw on require('./path.mjs');
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This is an extremely important part of the ESM implementation
that should have been unflagged as a breaking change in v12.0.0
to allow us to unflag ESM in Node.js 12.x before LTS. Assuming we
can get consensus on this behavior I would argue that this Semver-Major
behavior change could be viewed as a Semver-Patch fix in v12.0.1

PR-URL: https://github.com/nodejs/node/pull/27417
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Jan Krems <jan.krems@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Ujjwal Sharma <usharma1998@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
---
 doc/api/modules.md                 |  9 +++++++++
 lib/internal/modules/cjs/loader.js |  8 +++-----
 test/parallel/test-require-mjs.js  | 11 +++++++++++
 3 files changed, 23 insertions(+), 5 deletions(-)
 create mode 100644 test/parallel/test-require-mjs.js

diff --git a/doc/api/modules.md b/doc/api/modules.md
index 4acead178e4093..ca469d635ed272 100644
--- a/doc/api/modules.md
+++ b/doc/api/modules.md
@@ -132,6 +132,13 @@ variable. Since the module lookups using `node_modules` folders are all
 relative, and based on the real path of the files making the calls to
 `require()`, the packages themselves can be anywhere.
 
+## Addenda: The .mjs extension
+
+It is not possible to `require()` files that have the `.mjs` extension.
+Attempting to do so will throw [an error][]. The `.mjs` extension is
+reserved for [ECMAScript Modules][] which cannot be loaded via `require()`.
+See [ECMAScript Modules][] for more details.
+
 ## All Together...
 
 <!-- type=misc -->
@@ -950,6 +957,8 @@ requireUtil('./some-tool');
 [`createRequire()`]: #modules_module_createrequire_filename
 [`module` object]: #modules_the_module_object
 [`path.dirname()`]: path.html#path_path_dirname_path
+[ECMAScript Modules]: esm.html
+[an error]: errors.html#errors_err_require_esm
 [exports shortcut]: #modules_exports_shortcut
 [module resolution]: #modules_all_together
 [module wrapper]: #modules_the_module_wrapper
diff --git a/lib/internal/modules/cjs/loader.js b/lib/internal/modules/cjs/loader.js
index ff1128e903dc2e..91b89860e7b66d 100644
--- a/lib/internal/modules/cjs/loader.js
+++ b/lib/internal/modules/cjs/loader.js
@@ -800,11 +800,9 @@ Module._extensions['.node'] = function(module, filename) {
   return process.dlopen(module, path.toNamespacedPath(filename));
 };
 
-if (experimentalModules) {
-  Module._extensions['.mjs'] = function(module, filename) {
-    throw new ERR_REQUIRE_ESM(filename);
-  };
-}
+Module._extensions['.mjs'] = function(module, filename) {
+  throw new ERR_REQUIRE_ESM(filename);
+};
 
 // Bootstrap main module.
 Module.runMain = function() {
diff --git a/test/parallel/test-require-mjs.js b/test/parallel/test-require-mjs.js
new file mode 100644
index 00000000000000..69f8527555db71
--- /dev/null
+++ b/test/parallel/test-require-mjs.js
@@ -0,0 +1,11 @@
+'use strict';
+require('../common');
+const assert = require('assert');
+
+assert.throws(
+  () => require('../fixtures/es-modules/test-esm-ok.mjs'),
+  {
+    message: /Must use import to load ES Module/,
+    code: 'ERR_REQUIRE_ESM'
+  }
+);