module: fix legacy node specifier resolution to resolve "main" field

lib/internal/modules/esm/resolve.js

@@ -34,7 +34,7 @@ const { getOptionValue } = require('internal/options');
 const policy = getOptionValue('--experimental-policy') ?
   require('internal/process/policy') :
   null;
-const { sep, relative } = require('path');
+const { sep, relative, resolve } = require('path');
 const preserveSymlinks = getOptionValue('--preserve-symlinks');
 const preserveSymlinksMain = getOptionValue('--preserve-symlinks-main');
 const typeFlag = getOptionValue('--input-type');
@@ -204,16 +204,18 @@ function getPackageScopeConfig(resolved) {
   return packageConfig;
 }
 
-/*
+/**
  * Legacy CommonJS main resolution:
  * 1. let M = pkg_url + (json main field)
  * 2. TRY(M, M.js, M.json, M.node)
  * 3. TRY(M/index.js, M/index.json, M/index.node)
  * 4. TRY(pkg_url/index.js, pkg_url/index.json, pkg_url/index.node)
  * 5. NOT_FOUND
+ * @param {string | URL} url
+ * @returns {boolean}
  */
 function fileExists(url) {
-  return tryStatSync(fileURLToPath(url)).isFile();
+  return statSync(url, { throwIfNoEntry: false })?.isFile() ?? false;
 }
 
 function legacyMainResolve(packageJSONUrl, packageConfig, base) {
@@ -272,7 +274,19 @@ function resolveExtensions(search) {
   return undefined;
 }
 
-function resolveIndex(search) {
+function resolveDirectoryEntry(search) {
+  const dirPath = fileURLToPath(search);
+  const pkgJsonPath = resolve(dirPath, 'package.json');
+  if (fileExists(pkgJsonPath)) {
+    const pkgJson = packageJsonReader.read(pkgJsonPath);
+    if (pkgJson.containsKeys) {
+      const { main } = JSONParse(pkgJson.string);
+      if (main != null) {
+        const mainUrl = pathToFileURL(resolve(dirPath, main));
+        return resolveExtensionsWithTryExactName(mainUrl);
+      }
+    }
+  }
   return resolveExtensions(new URL('index', search));
 }
 
@@ -288,10 +302,10 @@ function finalizeResolution(resolved, base) {
     let file = resolveExtensionsWithTryExactName(resolved);
     if (file !== undefined) return file;
     if (!StringPrototypeEndsWith(path, '/')) {
-      file = resolveIndex(new URL(`${resolved}/`));
+      file = resolveDirectoryEntry(new URL(`${resolved}/`));
       if (file !== undefined) return file;
     } else {
-      return resolveIndex(resolved) || resolved;
+      return resolveDirectoryEntry(resolved) || resolved;
     }
     throw new ERR_MODULE_NOT_FOUND(
       resolved.pathname, fileURLToPath(base), 'module');

test/es-module/test-esm-specifiers-legacy-flag.mjs

@@ -6,12 +6,15 @@ import assert from 'assert';
 import commonjs from '../fixtures/es-module-specifiers/package-type-commonjs';
 // esm index.js
 import module from '../fixtures/es-module-specifiers/package-type-module';
+// Directory entry with main.js
+import main from '../fixtures/es-module-specifiers/dir-with-main';
 // Notice the trailing slash
 import success, { explicit, implicit, implicitModule }
   from '../fixtures/es-module-specifiers/';
 
 assert.strictEqual(commonjs, 'commonjs');
 assert.strictEqual(module, 'module');
+assert.strictEqual(main, 'main');
 assert.strictEqual(success, 'success');
 assert.strictEqual(explicit, 'esm');
 assert.strictEqual(implicit, 'cjs');

test/fixtures/es-module-specifiers/dir-with-main/main.js

@@ -0,0 +1 @@
+module.exports = 'main';

test/fixtures/es-module-specifiers/dir-with-main/package.json

@@ -0,0 +1,3 @@
+{
+    "main": "./main.js"
+}