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,