From 3b1795285b5b5db9f7831307671ea59cd2f2d137 Mon Sep 17 00:00:00 2001
From: Paul Bouchon <mail@bitpshr.net>
Date: Mon, 10 Aug 2026 17:14:01 -0400
Subject: [PATCH] module: fix --check on ambiguous ESM files

A `.js` file with no `"type"` in the nearest package.json has no format
of its own, and `defaultGetFormat()` reports it as null. `--check`
passed that null straight to `wrapSafe()`, which parses as CommonJS.
Module syntax makes that parse bail out early, so the file was reported
as valid and `--check` exited 0 even though it is not valid JavaScript
under either goal.

At load time the goal for such a file is decided by looking for module
syntax in the source. Decide it the same way here, so the file is
parsed as a module and its real syntax error is reported. Files whose
format is known are unaffected, as are ambiguous files without module
syntax, which are still parsed as CommonJS.

Fixes: https://github.com/nodejs/node/issues/65202
Signed-off-by: Paul Bouchon <mail@bitpshr.net>
---
 lib/internal/main/check_syntax.js                | 12 ++++++++++++
 test/fixtures/syntax/bad_syntax_esm_ambiguous.js |  2 ++
 test/sequential/test-cli-syntax-bad.js           |  3 +++
 3 files changed, 17 insertions(+)
 create mode 100644 test/fixtures/syntax/bad_syntax_esm_ambiguous.js

diff --git a/lib/internal/main/check_syntax.js b/lib/internal/main/check_syntax.js
index 16e367c4e089..7bc7bbc1ede4 100644
--- a/lib/internal/main/check_syntax.js
+++ b/lib/internal/main/check_syntax.js
@@ -67,6 +67,18 @@ async function checkSyntax(source, filename) {
     format = await defaultGetFormat(new URL(url));
   }
 
+  // A `.js` file with no `"type"` in the nearest package.json has no format of
+  // its own. At load time the goal is decided by looking for module syntax in
+  // the source, so decide it the same way here. Otherwise such a file is only
+  // ever parsed as CommonJS, where module syntax makes the parse bail out
+  // before any syntax error in the rest of the file is reported.
+  if (format === null || format === undefined) {
+    const { containsModuleSyntax } = internalBinding('contextify');
+    if (containsModuleSyntax(source, filename)) {
+      format = 'module';
+    }
+  }
+
   if (format === 'module') {
     const { ModuleWrap } = internalBinding('module_wrap');
     new ModuleWrap(filename, undefined, source, 0, 0);
diff --git a/test/fixtures/syntax/bad_syntax_esm_ambiguous.js b/test/fixtures/syntax/bad_syntax_esm_ambiguous.js
new file mode 100644
index 000000000000..511f42994f90
--- /dev/null
+++ b/test/fixtures/syntax/bad_syntax_esm_ambiguous.js
@@ -0,0 +1,2 @@
+import fs from 'node:fs';
+var = ;
diff --git a/test/sequential/test-cli-syntax-bad.js b/test/sequential/test-cli-syntax-bad.js
index e967ff36ac28..0cf9d020b30f 100644
--- a/test/sequential/test-cli-syntax-bad.js
+++ b/test/sequential/test-cli-syntax-bad.js
@@ -21,6 +21,9 @@ const syntaxErrorRE = /^SyntaxError: \b/m;
   'syntax/bad_syntax',
   'syntax/bad_syntax_shebang.js',
   'syntax/bad_syntax_shebang',
+  // A `.js` file with no `"type"` in the nearest package.json, whose module
+  // syntax makes it load as ESM. Refs: https://github.com/nodejs/node/issues/65202
+  'syntax/bad_syntax_esm_ambiguous.js',
 ].forEach((file) => {
   const path = fixtures.path(file);