module: throw error for ESM syntax in explicit commonjs entry

When a main entry point contains ESM syntax but is in a package with
"type": "commonjs" in package.json, the module would silently exit
with code 0 without executing or showing any error. This happened
because the CJS loader detected ESM syntax, attempted to defer to
ESM loading, but the async execution never completed before the
process exited.

This change throws a SyntaxError for main modules with ESM syntax
in explicitly CommonJS-typed packages, matching the error Node.js
throws when import syntax appears in other CommonJS contexts.

Fixes: #61104

Author: heathdutton

lib/internal/modules/cjs/loader.js

@@ -1751,6 +1751,7 @@ function wrapSafe(filename, content, cjsModuleInstance, format) {
  * @returns {any}
  */
 Module.prototype._compile = function(content, filename, format) {
+  const explicitCommonJS = format === 'commonjs' || format === 'commonjs-typescript';
   if (format === 'commonjs-typescript' || format === 'module-typescript' || format === 'typescript') {
     content = stripTypeScriptModuleTypes(content, filename);
     switch (format) {
@@ -1777,6 +1778,10 @@ Module.prototype._compile = function(content, filename, format) {
     const result = wrapSafe(filename, content, this, format);
     compiledWrapper = result.function;
     if (result.canParseAsESM) {
+      // Throw for ESM syntax in explicitly CommonJS main entry to avoid silent failure.
+      if (explicitCommonJS && this[kIsMainSymbol]) {
+        throw new SyntaxError('Cannot use import statement outside a module');
+      }
       format = 'module';
     }
   }

test/es-module/test-esm-syntax-in-cjs-main.js

@@ -0,0 +1,31 @@
+// Flags: --no-warnings
+'use strict';
+
+// Test that running a main entry point with ESM syntax in a "type": "commonjs"
+// package throws an error instead of silently failing with exit code 0.
+// Regression test for https://github.com/nodejs/node/issues/61104
+
+const { spawnPromisified } = require('../common');
+const fixtures = require('../common/fixtures.js');
+const assert = require('node:assert');
+const { execPath } = require('node:process');
+const { describe, it } = require('node:test');
+
+describe('ESM syntax in explicit CommonJS main entry point', { concurrency: !process.env.TEST_PARALLEL }, () => {
+  it('should throw SyntaxError when main module has ESM syntax in type:commonjs package', async () => {
+    const mainScript = fixtures.path('es-modules/package-type-commonjs-esm-syntax/esm-script.js');
+    const { code, signal, stderr } = await spawnPromisified(execPath, [mainScript]);
+
+    // Should exit with non-zero exit code
+    assert.strictEqual(code, 1, `Expected exit code 1, got ${code}`);
+    assert.strictEqual(signal, null);
+
+    // Should contain SyntaxError about import statement
+    assert.match(stderr, /SyntaxError/,
+      'Expected error to be a SyntaxError');
+    assert.match(stderr, /Cannot use import statement outside a module/,
+      'Expected error message about import statement');
+    assert.match(stderr, /esm-script\.js/,
+      'Expected error message to mention the script filename');
+  });
+});

test/fixtures/es-modules/package-type-commonjs-esm-syntax/esm-script.js

@@ -0,0 +1,4 @@
+// This file has ESM syntax but is in a "type": "commonjs" package
+console.log('script STARTED');
+import { version } from 'node:process';
+console.log(version);

test/fixtures/es-modules/package-type-commonjs-esm-syntax/package.json

@@ -0,0 +1,3 @@
+{
+  "type": "commonjs"
+}