module: warn on require of .js inside type: module

PR-URL: #29909
Reviewed-By: Jan Krems <jan.krems@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

lib/internal/modules/cjs/loader.js

@@ -261,7 +261,10 @@ function readPackageScope(checkPath) {
     if (checkPath.endsWith(path.sep + 'node_modules'))
       return false;
     const pjson = readPackage(checkPath);
-    if (pjson) return pjson;
+    if (pjson) return {
+      path: checkPath,
+      data: pjson
+    };
   }
   return false;
 }
@@ -960,13 +963,32 @@ Module.prototype._compile = function(content, filename) {
   return result;
 };
 
-
 // Native extension for .js
+let warnRequireESM = true;
 Module._extensions['.js'] = function(module, filename) {
-  if (experimentalModules && filename.endsWith('.js')) {
+  if (filename.endsWith('.js')) {
     const pkg = readPackageScope(filename);
-    if (pkg && pkg.type === 'module') {
-      throw new ERR_REQUIRE_ESM(filename);
+    if (pkg && pkg.data && pkg.data.type === 'module') {
+      if (warnRequireESM) {
+        const parentPath = module.parent && module.parent.filename;
+        const basename = parentPath &&
+            path.basename(filename) === path.basename(parentPath) ?
+          filename : path.basename(filename);
+        process.emitWarning(
+          'require() of ES modules is not supported.\nrequire() of ' +
+          `${filename} ${parentPath ? `from ${module.parent.filename} ` : ''}` +
+          'is an ES module file as it is a .js file whose nearest parent ' +
+          'package.json contains "type": "module" which defines all .js ' +
+          'files in that package scope as ES modules.\nInstead rename ' +
+          `${basename} to end in .cjs, change the requiring code to use ` +
+          'import(), or remove "type": "module" from ' +
+          `${path.resolve(pkg.path, 'package.json')}.`
+        );
+        warnRequireESM = false;
+      }
+      if (experimentalModules) {
+        throw new ERR_REQUIRE_ESM(filename);
+      }
     }
   }
   const content = fs.readFileSync(filename, 'utf8');

test/es-module/test-cjs-esm-warn.js

@@ -0,0 +1,38 @@
+'use strict';
+
+const common = require('../common');
+const fixtures = require('../common/fixtures');
+const { spawn } = require('child_process');
+const assert = require('assert');
+const path = require('path');
+
+const requiring = path.resolve(fixtures.path('/es-modules/cjs-esm.js'));
+const required = path.resolve(
+  fixtures.path('/es-modules/package-type-module/cjs.js')
+);
+const pjson = path.resolve(
+  fixtures.path('/es-modules/package-type-module/package.json')
+);
+
+const basename = 'cjs.js';
+
+const child = spawn(process.execPath, [requiring]);
+let stderr = '';
+child.stderr.setEncoding('utf8');
+child.stderr.on('data', (data) => {
+  stderr += data;
+});
+child.on('close', common.mustCall((code, signal) => {
+  assert.strictEqual(code, 0);
+  assert.strictEqual(signal, null);
+
+  assert.strictEqual(stderr, `(node:${child.pid}) Warning: ` +
+    'require() of ES modules is not supported.\nrequire() of ' +
+    `${required} from ${requiring} ` +
+    'is an ES module file as it is a .js file whose nearest parent ' +
+    'package.json contains "type": "module" which defines all .js ' +
+    'files in that package scope as ES modules.\nInstead rename ' +
+    `${basename} to end in .cjs, change the requiring code to use ` +
+    'import(), or remove "type": "module" from ' +
+    `${pjson}.\n`);
+}));

test/fixtures/es-modules/cjs-esm.js

@@ -0,0 +1 @@
+require('./package-type-module/cjs.js');

test/fixtures/es-modules/package-type-module/cjs.js

@@ -0,0 +1 @@
+module.exports = 'asdf';