esm: improve check for ESM syntax

Author: GeoffreyBooth

lib/internal/modules/cjs/loader.js

@@ -98,7 +98,7 @@ const {
 const {
   getCjsConditions,
   initializeCjsConditions,
-  hasEsmSyntax,
+  isModuleSyntaxError,
   loadBuiltinModule,
   makeRequireFunction,
   normalizeReferrerURL,
@@ -1393,10 +1393,20 @@ Module._extensions['.js'] = function(module, filename) {
     const pkg = packageJsonReader.readPackageScope(filename) || { __proto__: null };
     // Function require shouldn't be used in ES modules.
     if (pkg.data?.type === 'module') {
+      // This is an error path, because `require` of a `.js` file in a `"type": "module"` scope is not allowed.
       const parent = moduleParentCache.get(module);
       const parentPath = parent?.filename;
       const packageJsonPath = path.resolve(pkg.path, 'package.json');
-      const usesEsm = hasEsmSyntax(content);
+
+      let usesEsm = false;
+      try {
+        internalCompileFunction(content, ['exports', 'require', 'module', '__filename', '__dirname'], { filename });
+      } catch (compilationError) {
+        if (isModuleSyntaxError(compilationError)) {
+          usesEsm = true;
+        }
+      }
+
       const err = new ERR_REQUIRE_ESM(filename, usesEsm, parentPath,
                                       packageJsonPath);
       // Attempt to reconstruct the parent require frame.

lib/internal/modules/esm/translators.js

@@ -4,7 +4,6 @@ const {
   ArrayPrototypeMap,
   Boolean,
   JSONParse,
-  ObjectGetPrototypeOf,
   ObjectPrototypeHasOwnProperty,
   ObjectKeys,
   ReflectApply,
@@ -15,7 +14,6 @@ const {
   StringPrototypeReplaceAll,
   StringPrototypeSlice,
   StringPrototypeStartsWith,
-  SyntaxErrorPrototype,
   globalThis: { WebAssembly },
 } = primordials;
 
@@ -33,7 +31,7 @@ const assert = require('internal/assert');
 const { readFileSync } = require('fs');
 const { dirname, extname, isAbsolute } = require('path');
 const {
-  hasEsmSyntax,
+  isModuleSyntaxError,
   loadBuiltinModule,
   stripBOM,
 } = require('internal/modules/helpers');
@@ -168,8 +166,7 @@ translators.set('module', async function moduleStrategy(url, source, isMain) {
  * @param {string} [filename] Useful only if `content` is unknown.
  */
 function enrichCJSError(err, content, filename) {
-  if (err != null && ObjectGetPrototypeOf(err) === SyntaxErrorPrototype &&
-      hasEsmSyntax(content || readFileSync(filename, 'utf-8'))) {
+  if (isModuleSyntaxError(err)) {
     // Emit the warning synchronously because we are in the middle of handling
     // a SyntaxError that will throw and likely terminate the process before an
     // asynchronous warning would be emitted.

lib/internal/modules/helpers.js

@@ -3,15 +3,16 @@
 const {
   ArrayPrototypeForEach,
   ArrayPrototypeJoin,
-  ArrayPrototypeSome,
   ObjectDefineProperty,
+  ObjectGetPrototypeOf,
   ObjectPrototypeHasOwnProperty,
   SafeMap,
   SafeSet,
   StringPrototypeCharCodeAt,
   StringPrototypeIncludes,
   StringPrototypeSlice,
   StringPrototypeStartsWith,
+  SyntaxErrorPrototype,
 } = primordials;
 const {
   ERR_INVALID_ARG_TYPE,
@@ -300,31 +301,28 @@ function normalizeReferrerURL(referrer) {
 }
 
 /**
- * For error messages only, check if ESM syntax is in use.
- * @param {string} code
+ * Check if the error is a syntax error due to ESM syntax in CommonJS.
+ * - `import` statements return an error with a message `Cannot use import statement outside a module`.
+ * - `export` statements return an error with a message `Unexpected token 'export'`.
+ * - `import.meta` returns an error with a message `Cannot use 'import.meta' outside a module`.
+ * Top-level `await` currently returns the same error message as when `await` is used in a sync function,
+ * so we don't use it as a disambiguation.
+ * Dynamic `import()` is permitted in CommonJS, so we don't use it as a disambiguation.
+ * @param {Error} err
  */
-function hasEsmSyntax(code) {
-  debug('Checking for ESM syntax');
-  const parser = require('internal/deps/acorn/acorn/dist/acorn').Parser;
-  let root;
-  try {
-    root = parser.parse(code, { sourceType: 'module', ecmaVersion: 'latest' });
-  } catch {
-    return false;
-  }
-
-  return ArrayPrototypeSome(root.body, (stmt) =>
-    stmt.type === 'ExportDefaultDeclaration' ||
-    stmt.type === 'ExportNamedDeclaration' ||
-    stmt.type === 'ImportDeclaration' ||
-    stmt.type === 'ExportAllDeclaration');
+function isModuleSyntaxError(err) {
+  return err != null && ObjectGetPrototypeOf(err) === SyntaxErrorPrototype && (
+    err.message === 'Cannot use import statement outside a module' ||
+    err.message === "Unexpected token 'export'" ||
+    err.message === "Cannot use 'import.meta' outside a module"
+  );
 }
 
 module.exports = {
   addBuiltinLibsToObject,
   getCjsConditions,
   initializeCjsConditions,
-  hasEsmSyntax,
+  isModuleSyntaxError,
   loadBuiltinModule,
   makeRequireFunction,
   normalizeReferrerURL,