review findings part 1. split into two PRs will follow

Author: dygabo

lib/internal/modules/esm/get_format.js

@@ -22,6 +22,8 @@ const experimentalNetworkImports =
 const { containsModuleSyntax } = internalBinding('contextify');
 const { getPackageScopeConfig, getPackageType } = require('internal/modules/package_json_reader');
 const { fileURLToPath } = require('internal/url');
+const { readFileSync } = require('fs');
+const { Buffer } = require('buffer');
 const { ERR_UNKNOWN_FILE_EXTENSION } = require('internal/errors').codes;
 
 const protocolHandlers = {
@@ -82,6 +84,24 @@ function underNodeModules(url) {
   return StringPrototypeIncludes(url.pathname, '/node_modules/');
 }
 
+/**
+ * Determine whether the given source contains CJS or ESM module syntax.
+ * @param {string} source
+ * @param {URL} url
+ */
+function detectModuleFormat(source, url) {
+  try {
+    let realSource = source ?? readFileSync(url, 'utf8');
+    if (Buffer.isBuffer(realSource)) {
+      // `containsModuleSyntax` requires source to be passed in as string
+      realSource = realSource.toString();
+    }
+    return containsModuleSyntax(realSource, fileURLToPath(url), url) ? 'module' : 'commonjs';
+  } catch {
+    return 'commonjs';
+  }
+}
+
 let typelessPackageJsonFilesWarnedAbout;
 /**
  * @param {URL} url
@@ -92,12 +112,6 @@ let typelessPackageJsonFilesWarnedAbout;
 function getFileProtocolModuleFormat(url, context = { __proto__: null }, ignoreErrors) {
   const { source } = context;
   const ext = extname(url);
-  const deduceFormat = (fromSource, fromUrl) => {
-    const { existsSync, readFileSync } = require('fs');
-    const realSource = fromSource ?? existsSync(fromUrl) ? readFileSync(fromUrl).toString() : undefined;
-    return realSource ? // Do we have a source? check for module syntax
-      (containsModuleSyntax(realSource, fileURLToPath(fromUrl), fromUrl) ? 'module' : 'commonjs') : 'commonjs';
-  };
 
   if (ext === '.js') {
     const { type: packageType, pjsonPath } = getPackageScopeConfig(url);
@@ -119,7 +133,7 @@ function getFileProtocolModuleFormat(url, context = { __proto__: null }, ignoreE
         // `source` is undefined when this is called from `defaultResolve`;
         // but this gets called again from `defaultLoad`/`defaultLoadSync`.
         if (getOptionValue('--experimental-detect-module')) {
-          const format = deduceFormat(source, url);
+          const format = detectModuleFormat(source, url);
           if (format === 'module') {
             // This module has a .js extension, a package.json with no `type` field, and ESM syntax.
             // Warn about the missing `type` field so that the user can avoid the performance penalty of detection.
@@ -160,7 +174,7 @@ function getFileProtocolModuleFormat(url, context = { __proto__: null }, ignoreE
       default: { // The user did not pass `--experimental-default-type`.
         if (getOptionValue('--experimental-detect-module')) {
           const format = getFormatOfExtensionlessFile(url);
-          return (format === 'module') ? deduceFormat(source, url) : format;
+          return (format === 'module') ? detectModuleFormat(source, url) : format;
         }
         return 'commonjs';
       }

test/es-module/test-esm-cjs-exports.js

@@ -28,6 +28,6 @@ describe('ESM: importing CJS', { concurrency: !process.env.TEST_PARALLEL }, () =
     assert.strictEqual(code, 0);
     assert.strictEqual(signal, null);
     assert.strictEqual(stdout, '');
-    assert.match(stderr, /[MODULE_TYPELESS_PACKAGE_JSON]/);
+    assert.match(stderr, /\[MODULE_TYPELESS_PACKAGE_JSON\]/);
   });
 });