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

PR-URL: https://github.com/nodejs/node/pull/29909
Reviewed-By: Jan Krems <jan.krems@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

1 files changed, 27 insertions, 5 deletions

diff --git a/lib/internal/modules/cjs/loader.js b/lib/internal/modules/cjs/loader.js
index 2d058b7e0d..64505220ab 100644
--- a/lib/internal/modules/cjs/loader.js
+++ b/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');