module: disable CommonJS support if entry point is ESM

lib/internal/modules/cjs/loader.js

@@ -63,15 +63,19 @@ const {
   StringPrototypeSlice,
   StringPrototypeSplit,
   StringPrototypeStartsWith,
+  Symbol,
 } = primordials;
 
+const kContainsESMSyntax = Symbol('contains ESM syntax');
+
 // Map used to store CJS parsing data.
 const cjsParseCache = new SafeWeakMap();
 
 // Set first due to cycle with ESM loader functions.
 module.exports = {
   wrapSafe, Module, toRealPath, readPackageScope, cjsParseCache,
-  get hasLoadedAnyUserCJSModule() { return hasLoadedAnyUserCJSModule; }
+  get hasLoadedAnyUserCJSModule() { return hasLoadedAnyUserCJSModule; },
+  kContainsESMSyntax,
 };
 
 const { NativeModule } = require('internal/bootstrap/loaders');

lib/internal/modules/esm/translators.js

@@ -36,7 +36,8 @@ const {
 } = require('internal/modules/cjs/helpers');
 const {
   Module: CJSModule,
-  cjsParseCache
+  cjsParseCache,
+  kContainsESMSyntax,
 } = require('internal/modules/cjs/loader');
 const internalURLModule = require('internal/url');
 const { defaultGetSource } = require(
@@ -171,6 +172,7 @@ function enrichCJSError(err) {
       'To load an ES module, set "type": "module" in the package.json or use ' +
       'the .mjs extension.'
     );
+    err[kContainsESMSyntax] = true;
   }
 }
 

lib/internal/modules/run_main.js

@@ -4,7 +4,7 @@ const {
   StringPrototypeEndsWith,
 } = primordials;
 const CJSLoader = require('internal/modules/cjs/loader');
-const { Module, toRealPath, readPackageScope } = CJSLoader;
+const { Module, kContainsESMSyntax, toRealPath, readPackageScope } = CJSLoader;
 const { getOptionValue } = require('internal/options');
 const path = require('path');
 
@@ -75,8 +75,25 @@ function executeUserEntryPoint(main = process.argv[1]) {
   if (useESMLoader) {
     runMainESM(resolvedMain || main);
   } else {
-    // Module._load is the monkey-patchable CJS module loader.
-    Module._load(main, null, true);
+    try {
+      // Module._load is the monkey-patchable CJS module loader.
+      Module._load(main, null, true);
+    } catch (err) {
+      if (err?.[kContainsESMSyntax] && !readPackageScope(resolvedMain)) {
+        const { emitWarning } = require('internal/process/warning');
+        emitWarning(`No package.json detected, attempting to load "${main}" with CommonJS support disabled.`);
+        const esmLoader = require('internal/process/esm_loader');
+        esmLoader.ESMLoader._getFormat = (url, _, defaultGetFormat) => {
+          const defaultFormat = defaultGetFormat(url);
+          if (defaultFormat.format === 'commonjs')
+            defaultFormat.format = 'module';
+          return defaultFormat;
+        };
+        runMainESM(resolvedMain || main);
+      } else {
+        throw err;
+      }
+    }
   }
 }
 

test/message/esm_syntax_in_entry_point_without_package.json.js

@@ -0,0 +1 @@
+export {};

test/message/esm_syntax_in_entry_point_without_package.json.out

@@ -0,0 +1,3 @@
+(node:*) Warning: To load an ES module, set "type": "module" in the package.json or use the .mjs extension.
+(Use `node --trace-warnings ...` to show where the warning was created)
+(node:*) Warning: No package.json detected, attempting to load "*/test/message/esm_syntax_in_entry_point_without_package.json.js" with CommonJS support disabled.