rewrite based on review comments and defaultGetFormat implementation

Author: dygabo

lib/internal/modules/esm/resolve.js

@@ -37,7 +37,7 @@ const { getOptionValue } = require('internal/options');
 const policy = getOptionValue('--experimental-policy') ?
   require('internal/process/policy') :
   null;
-const { sep, relative, resolve } = require('path');
+const { sep, relative, resolve, extname } = require('path');
 const preserveSymlinks = getOptionValue('--preserve-symlinks');
 const preserveSymlinksMain = getOptionValue('--preserve-symlinks-main');
 const typeFlag = getOptionValue('--input-type');
@@ -62,6 +62,9 @@ const userConditions = getOptionValue('--conditions');
 const noAddons = getOptionValue('--no-addons');
 const addonConditions = noAddons ? [] : ['node-addons'];
 
+const experimentalSpecifierResolution =
+  getOptionValue('--experimental-specifier-resolution');
+
 const DEFAULT_CONDITIONS = ObjectFreeze([
   'node',
   'import',
@@ -467,11 +470,19 @@ function resolvePackageTargetString(
   const composeResult = (resolved) => {
     let format;
     try {
-      // Extension has higher priority than package.json type descriptor
-      if (StringPrototypeEndsWith(resolved.href, '.mjs')) {
-        format = 'module';
-      } else {
+      const ext = extname(resolved.pathname);
+      if (ext === '.js') {
         format = getPackageType(resolved);
+      } else {
+        format = extensionFormatMap[ext];
+      }
+      if (!format) {
+        if (experimentalSpecifierResolution === 'node') {
+          process.emitWarning(
+            'The Node.js specifier resolution in ESM is experimental.',
+            'ExperimentalWarning');
+          format = legacyExtensionFormatMap[ext];
+        }
       }
     } catch (err) {
       if (err.code === 'ERR_INVALID_FILE_URL_PATH') {
@@ -1112,4 +1123,6 @@ module.exports = {
 };
 
 // cycle
-const { defaultGetFormat } = require('internal/modules/esm/get_format');
+const { defaultGetFormat,
+        extensionFormatMap,
+        legacyExtensionFormatMap } = require('internal/modules/esm/get_format');