From 9842d3e1a74f8d8ef1a6fec1190fae27d076a2be Mon Sep 17 00:00:00 2001
From: Matteo Collina <hello@matteocollina.com>
Date: Mon, 9 Mar 2026 13:23:37 +0100
Subject: [PATCH] module: fix extensionless CJS files in type:module packages

PR #61600 made the CJS loader respect the package.json type field for
extensionless files, which fixed #61104 but also enforced type: "module"
for extensionless files. This broke CJS extensionless files inside ESM
packages (e.g. yargs v17).

Only enforce type: "commonjs" for extensionless files. For type: "module",
leave format undefined so syntax detection handles it. This restores the
documented exception in the CJS documentation that has existed since v16:
extensionless files in type: "module" packages are recognized as CJS when
included via require().

Fixes: https://github.com/nodejs/node/issues/61971
Refs: https://github.com/yargs/yargs/issues/2509
Refs: https://github.com/nodejs/node/pull/61600
PR-URL: https://github.com/nodejs/node/pull/62083
---
 lib/internal/modules/cjs/loader.js                     |  8 ++++----
 test/es-module/test-extensionless-esm-type-commonjs.js | 10 ++++++++++
 .../fixtures/es-modules/extensionless-cjs-module/index |  1 +
 .../es-modules/extensionless-cjs-module/package.json   |  3 +++
 4 files changed, 18 insertions(+), 4 deletions(-)
 create mode 100644 test/fixtures/es-modules/extensionless-cjs-module/index
 create mode 100644 test/fixtures/es-modules/extensionless-cjs-module/package.json

diff --git a/lib/internal/modules/cjs/loader.js b/lib/internal/modules/cjs/loader.js
index f5a47448ba3eb9..6e82be7cf3e7ee 100644
--- a/lib/internal/modules/cjs/loader.js
+++ b/lib/internal/modules/cjs/loader.js
@@ -1952,12 +1952,12 @@ Module._extensions['.js'] = function(module, filename) {
       format = 'typescript';
     }
   } else if (path.extname(filename) === '') {
-    // Extensionless files skip the .js suffix check above. When type is explicit,
-    // follow it so ESM syntax surfaces as SyntaxError for commonjs instead of
-    // silently delegating to ESM.
+    // Extensionless files skip the .js suffix check above. When type is commonjs, follow it so ESM
+    // syntax surfaces as SyntaxError. For type: module, leave format undefined so our syntax
+    // detection handles it (allowing CJS extensionless files in ESM packages).
     pkg = packageJsonReader.getNearestParentPackageJSON(filename);
     const typeFromPjson = pkg?.data?.type;
-    if (typeFromPjson === 'commonjs' || typeFromPjson === 'module') {
+    if (typeFromPjson === 'commonjs') {
       format = typeFromPjson;
     }
   }
diff --git a/test/es-module/test-extensionless-esm-type-commonjs.js b/test/es-module/test-extensionless-esm-type-commonjs.js
index cdfdf9361393e9..13349ed1c156af 100644
--- a/test/es-module/test-extensionless-esm-type-commonjs.js
+++ b/test/es-module/test-extensionless-esm-type-commonjs.js
@@ -17,3 +17,13 @@ spawnSyncAndAssert(process.execPath, [
   stdout: /script STARTED[\s\S]*v\d+\./,
   trim: true,
 });
+
+// CJS extensionless file inside a type: "module" package should work
+// when require()'d. Regression test for https://github.com/nodejs/node/issues/61971
+spawnSyncAndAssert(process.execPath, [
+  '-e', `const m = require(${JSON.stringify(
+    fixtures.path('es-modules', 'extensionless-cjs-module', 'index')
+  )}); if (m.hello !== 'world') throw new Error('expected CJS exports, got: ' + JSON.stringify(m))`,
+], {
+  status: 0,
+});
diff --git a/test/fixtures/es-modules/extensionless-cjs-module/index b/test/fixtures/es-modules/extensionless-cjs-module/index
new file mode 100644
index 00000000000000..6c45e12abc39f5
--- /dev/null
+++ b/test/fixtures/es-modules/extensionless-cjs-module/index
@@ -0,0 +1 @@
+module.exports = { hello: 'world' };
diff --git a/test/fixtures/es-modules/extensionless-cjs-module/package.json b/test/fixtures/es-modules/extensionless-cjs-module/package.json
new file mode 100644
index 00000000000000..3dbc1ca591c055
--- /dev/null
+++ b/test/fixtures/es-modules/extensionless-cjs-module/package.json
@@ -0,0 +1,3 @@
+{
+  "type": "module"
+}