module: support main w/o extension, pjson cache

This adds support for ensuring that the top-level main into Node is
supported loading when it has no extension for backwards-compat with
NodeJS bin workflows.

In addition package.json caching is implemented in the module lookup
process.

PR-URL: https://github.com/nodejs/node/pull/18728
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>

doc/api/esm.md

@@ -117,9 +117,12 @@ The resolve hook returns the resolved file URL and module format for a
 given module specifier and parent file URL:
 
 ```js
-import url from 'url';
+const baseURL = new URL('file://');
+baseURL.pathname = process.cwd() + '/';
 
-export async function resolve(specifier, parentModuleURL, defaultResolver) {
+export async function resolve(specifier,
+                              parentModuleURL = baseURL,
+                              defaultResolver) {
   return {
     url: new URL(specifier, parentModuleURL).href,
     format: 'esm'
@@ -127,7 +130,9 @@ export async function resolve(specifier, parentModuleURL, defaultResolver) {
 }
 ```
 
-The default NodeJS ES module resolution function is provided as a third
+The parentURL is provided as `undefined` when performing main Node.js load itself.
+
+The default Node.js ES module resolution function is provided as a third
 argument to the resolver for easy compatibility workflows.
 
 In addition to returning the resolved file URL value, the resolve hook also
@@ -155,7 +160,10 @@ import Module from 'module';
 const builtins = Module.builtinModules;
 const JS_EXTENSIONS = new Set(['.js', '.mjs']);
 
-export function resolve(specifier, parentModuleURL/*, defaultResolve */) {
+const baseURL = new URL('file://');
+baseURL.pathname = process.cwd() + '/';
+
+export function resolve(specifier, parentModuleURL = baseURL, defaultResolve) {
   if (builtins.includes(specifier)) {
     return {
       url: specifier,

lib/internal/loader/DefaultResolve.js

@@ -2,7 +2,6 @@
 
 const { URL } = require('url');
 const CJSmodule = require('module');
-const internalURLModule = require('internal/url');
 const internalFS = require('internal/fs');
 const NativeModule = require('native_module');
 const { extname } = require('path');
@@ -11,6 +10,7 @@ const preserveSymlinks = !!process.binding('config').preserveSymlinks;
 const errors = require('internal/errors');
 const { resolve: moduleWrapResolve } = internalBinding('module_wrap');
 const StringStartsWith = Function.call.bind(String.prototype.startsWith);
+const { getURLFromFilePath, getPathFromURL } = require('internal/url');
 
 const realpathCache = new Map();
 
@@ -57,7 +57,8 @@ function resolve(specifier, parentURL) {
 
   let url;
   try {
-    url = search(specifier, parentURL);
+    url = search(specifier,
+                 parentURL || getURLFromFilePath(`${process.cwd()}/`).href);
   } catch (e) {
     if (typeof e.message === 'string' &&
         StringStartsWith(e.message, 'Cannot find module'))
@@ -66,17 +67,27 @@ function resolve(specifier, parentURL) {
   }
 
   if (!preserveSymlinks) {
-    const real = realpathSync(internalURLModule.getPathFromURL(url), {
+    const real = realpathSync(getPathFromURL(url), {
       [internalFS.realpathCacheKey]: realpathCache
     });
     const old = url;
-    url = internalURLModule.getURLFromFilePath(real);
+    url = getURLFromFilePath(real);
     url.search = old.search;
     url.hash = old.hash;
   }
 
   const ext = extname(url.pathname);
-  return { url: `${url}`, format: extensionFormatMap[ext] || ext };
+
+  let format = extensionFormatMap[ext];
+  if (!format) {
+    const isMain = parentURL === undefined;
+    if (isMain)
+      format = 'cjs';
+    else
+      throw new errors.Error('ERR_UNKNOWN_FILE_EXTENSION', url.pathname);
+  }
+
+  return { url: `${url}`, format };
 }
 
 module.exports = resolve;

lib/internal/loader/Loader.js

@@ -1,51 +1,21 @@
 'use strict';
 
-const path = require('path');
-const { getURLFromFilePath, URL } = require('internal/url');
 const errors = require('internal/errors');
-
 const ModuleMap = require('internal/loader/ModuleMap');
 const ModuleJob = require('internal/loader/ModuleJob');
 const defaultResolve = require('internal/loader/DefaultResolve');
 const createDynamicModule = require('internal/loader/CreateDynamicModule');
 const translators = require('internal/loader/Translators');
-const { setImportModuleDynamicallyCallback } = internalBinding('module_wrap');
+
 const FunctionBind = Function.call.bind(Function.prototype.bind);
 
 const debug = require('util').debuglog('esm');
 
-// Returns a file URL for the current working directory.
-function getURLStringForCwd() {
-  try {
-    return getURLFromFilePath(`${process.cwd()}/`).href;
-  } catch (e) {
-    e.stack;
-    // If the current working directory no longer exists.
-    if (e.code === 'ENOENT') {
-      return undefined;
-    }
-    throw e;
-  }
-}
-
-function normalizeReferrerURL(referrer) {
-  if (typeof referrer === 'string' && path.isAbsolute(referrer)) {
-    return getURLFromFilePath(referrer).href;
-  }
-  return new URL(referrer).href;
-}
-
 /* A Loader instance is used as the main entry point for loading ES modules.
  * Currently, this is a singleton -- there is only one used for loading
  * the main module and everything in its dependency graph. */
 class Loader {
-  constructor(base = getURLStringForCwd()) {
+  constructor() {
-    if (typeof base !== 'string')
-      throw new errors.TypeError('ERR_INVALID_ARG_TYPE', 'base', 'string');
-
-    this.base = base;
-    this.isMain = true;
-
     // methods which translate input code or other information
     // into es modules
     this.translators = translators;
@@ -71,8 +41,9 @@ class Loader {
     this._dynamicInstantiate = undefined;
   }
 
-  async resolve(specifier, parentURL = this.base) {
+  async resolve(specifier, parentURL) {
-    if (typeof parentURL !== 'string')
+    const isMain = parentURL === undefined;
+    if (!isMain && typeof parentURL !== 'string')
       throw new errors.TypeError('ERR_INVALID_ARG_TYPE', 'parentURL', 'string');
 
     const { url, format } =
@@ -93,7 +64,7 @@ class Loader {
     return { url, format };
   }
 
-  async import(specifier, parent = this.base) {
+  async import(specifier, parent) {
     const job = await this.getModuleJob(specifier, parent);
     const module = await job.run();
     return module.namespace();
@@ -107,7 +78,7 @@ class Loader {
       this._dynamicInstantiate = FunctionBind(dynamicInstantiate, null);
   }
 
-  async getModuleJob(specifier, parentURL = this.base) {
+  async getModuleJob(specifier, parentURL) {
     const { url, format } = await this.resolve(specifier, parentURL);
     let job = this.moduleMap.get(url);
     if (job !== undefined)
@@ -134,24 +105,16 @@ class Loader {
     }
 
     let inspectBrk = false;
-    if (this.isMain) {
+    if (process._breakFirstLine) {
-      if (process._breakFirstLine) {
+      delete process._breakFirstLine;
-        delete process._breakFirstLine;
+      inspectBrk = true;
-        inspectBrk = true;
-      }
-      this.isMain = false;
     }
     job = new ModuleJob(this, url, loaderInstance, inspectBrk);
     this.moduleMap.set(url, job);
     return job;
   }
-
-  static registerImportDynamicallyCallback(loader) {
-    setImportModuleDynamicallyCallback(async (referrer, specifier) => {
-      return loader.import(specifier, normalizeReferrerURL(referrer));
-    });
-  }
 }
 
 Object.setPrototypeOf(Loader.prototype, null);
+
 module.exports = Loader;

lib/internal/loader/Translators.js

@@ -19,7 +19,7 @@ const JsonParse = JSON.parse;
 const translators = new SafeMap();
 module.exports = translators;
 
-// Stragety for loading a standard JavaScript module
+// Strategy for loading a standard JavaScript module
 translators.set('esm', async (url) => {
   const source = `${await readFileAsync(new URL(url))}`;
   debug(`Translating StandardModule ${url}`);
@@ -62,7 +62,7 @@ translators.set('builtin', async (url) => {
   });
 });
 
-// Stragety for loading a node native module
+// Strategy for loading a node native module
 translators.set('addon', async (url) => {
   debug(`Translating NativeModule ${url}`);
   return createDynamicModule(['default'], url, (reflect) => {
@@ -74,7 +74,7 @@ translators.set('addon', async (url) => {
   });
 });
 
-// Stragety for loading a JSON file
+// Strategy for loading a JSON file
 translators.set('json', async (url) => {
   debug(`Translating JSONModule ${url}`);
   return createDynamicModule(['default'], url, (reflect) => {

lib/internal/process/modules.js

@@ -1,17 +1,54 @@
 'use strict';
 
 const {
+  setImportModuleDynamicallyCallback,
   setInitializeImportMetaObjectCallback
 } = internalBinding('module_wrap');
 
+const { getURLFromFilePath } = require('internal/url');
+const Loader = require('internal/loader/Loader');
+const path = require('path');
+const { URL } = require('url');
+
+function normalizeReferrerURL(referrer) {
+  if (typeof referrer === 'string' && path.isAbsolute(referrer)) {
+    return getURLFromFilePath(referrer).href;
+  }
+  return new URL(referrer).href;
+}
+
 function initializeImportMetaObject(wrap, meta) {
   meta.url = wrap.url;
 }
 
-function setupModules() {
+let loaderResolve;
-  setInitializeImportMetaObjectCallback(initializeImportMetaObject);
+exports.loaderPromise = new Promise((resolve, reject) => {
-}
+  loaderResolve = resolve;
+});
 
-module.exports = {
+exports.ESMLoader = undefined;
-  setup: setupModules
+
+exports.setup = function() {
+  setInitializeImportMetaObjectCallback(initializeImportMetaObject);
+
+  let ESMLoader = new Loader();
+  const loaderPromise = (async () => {
+    const userLoader = process.binding('config').userLoader;
+    if (userLoader) {
+      const hooks = await ESMLoader.import(
+        userLoader, getURLFromFilePath(`${process.cwd()}/`).href);
+      ESMLoader = new Loader();
+      ESMLoader.hook(hooks);
+      exports.ESMLoader = ESMLoader;
+    }
+    return ESMLoader;
+  })();
+  loaderResolve(loaderPromise);
+
+  setImportModuleDynamicallyCallback(async (referrer, specifier) => {
+    const loader = await loaderPromise;
+    return loader.import(specifier, normalizeReferrerURL(referrer));
+  });
+
+  exports.ESMLoader = ESMLoader;
 };

lib/module.js

@@ -24,7 +24,6 @@
 const NativeModule = require('native_module');
 const util = require('util');
 const { decorateErrorStack } = require('internal/util');
-const internalModule = require('internal/module');
 const { getURLFromFilePath } = require('internal/url');
 const vm = require('vm');
 const assert = require('assert').ok;
@@ -35,6 +34,7 @@ const {
   internalModuleReadJSON,
   internalModuleStat
 } = process.binding('fs');
+const internalModule = require('internal/module');
 const preserveSymlinks = !!process.binding('config').preserveSymlinks;
 const experimentalModules = !!process.binding('config').experimentalModules;
 
@@ -43,10 +43,9 @@ const errors = require('internal/errors');
 module.exports = Module;
 
 // these are below module.exports for the circular reference
-const Loader = require('internal/loader/Loader');
+const internalESModule = require('internal/process/modules');
 const ModuleJob = require('internal/loader/ModuleJob');
 const createDynamicModule = require('internal/loader/CreateDynamicModule');
-let ESMLoader;
 
 function stat(filename) {
   filename = path.toNamespacedPath(filename);
@@ -447,7 +446,6 @@ Module._resolveLookupPaths = function(request, parent, newReturn) {
   return (newReturn ? parentDir : [id, parentDir]);
 };
 
-
 // Check the cache for the requested file.
 // 1. If a module already exists in the cache: return its exports object.
 // 2. If the module is native: call `NativeModule.require()` with the
@@ -460,22 +458,10 @@ Module._load = function(request, parent, isMain) {
     debug('Module._load REQUEST %s parent: %s', request, parent.id);
   }
 
-  if (isMain && experimentalModules) {
+  if (experimentalModules && isMain) {
-    (async () => {
+    internalESModule.loaderPromise.then((loader) => {
-      // loader setup
+      return loader.import(getURLFromFilePath(request).pathname);
-      if (!ESMLoader) {
+    })
-        ESMLoader = new Loader();
-        const userLoader = process.binding('config').userLoader;
-        if (userLoader) {
-          ESMLoader.isMain = false;
-          const hooks = await ESMLoader.import(userLoader);
-          ESMLoader = new Loader();
-          ESMLoader.hook(hooks);
-        }
-      }
-      Loader.registerImportDynamicallyCallback(ESMLoader);
-      await ESMLoader.import(getURLFromFilePath(request).pathname);
-    })()
     .catch((e) => {
       decorateErrorStack(e);
       console.error(e);
@@ -578,7 +564,8 @@ Module.prototype.load = function(filename) {
   Module._extensions[extension](this, filename);
   this.loaded = true;
 
-  if (ESMLoader) {
+  if (experimentalModules) {
+    const ESMLoader = internalESModule.ESMLoader;
     const url = getURLFromFilePath(filename);
     const urlString = `${url}`;
     const exports = this.exports;

src/env.h

@@ -54,7 +54,26 @@ class performance_state;
 
 namespace loader {
 class ModuleWrap;
-}
+
+struct Exists {
+  enum Bool { Yes, No };
+};
+
+struct IsValid {
+  enum Bool { Yes, No };
+};
+
+struct HasMain {
+  enum Bool { Yes, No };
+};
+
+struct PackageConfig {
+  const Exists::Bool exists;
+  const IsValid::Bool is_valid;
+  const HasMain::Bool has_main;
+  const std::string main;
+};
+}  // namespace loader
 
 // Pick an index that's hopefully out of the way when we're embedded inside
 // another application. Performance-wise or memory-wise it doesn't matter:
@@ -609,6 +628,8 @@ class Environment {
 
   std::unordered_multimap<int, loader::ModuleWrap*> module_map;
 
+  std::unordered_map<std::string, loader::PackageConfig> package_json_cache;
+
   inline double* heap_statistics_buffer() const;
   inline void set_heap_statistics_buffer(double* pointer);
 

src/module_wrap.cc

@@ -461,10 +461,9 @@ enum CheckFileOptions {
   CLOSE_AFTER_CHECK
 };
 
-Maybe<uv_file> CheckFile(const URL& search,
+Maybe<uv_file> CheckFile(const std::string& path,
                          CheckFileOptions opt = CLOSE_AFTER_CHECK) {
   uv_fs_t fs_req;
-  std::string path = search.ToFilePath();
   if (path.empty()) {
     return Nothing<uv_file>();
   }
@@ -481,19 +480,74 @@ Maybe<uv_file> CheckFile(const URL& search,
   uv_fs_req_cleanup(&fs_req);
 
   if (is_directory) {
-    uv_fs_close(nullptr, &fs_req, fd, nullptr);
+    CHECK_EQ(0, uv_fs_close(nullptr, &fs_req, fd, nullptr));
     uv_fs_req_cleanup(&fs_req);
     return Nothing<uv_file>();
   }
 
   if (opt == CLOSE_AFTER_CHECK) {
-    uv_fs_close(nullptr, &fs_req, fd, nullptr);
+    CHECK_EQ(0, uv_fs_close(nullptr, &fs_req, fd, nullptr));
     uv_fs_req_cleanup(&fs_req);
   }
 
   return Just(fd);
 }
 
+const PackageConfig& GetPackageConfig(Environment* env,
+                                                   const std::string path) {
+  auto existing = env->package_json_cache.find(path);
+  if (existing != env->package_json_cache.end()) {
+    return existing->second;
+  }
+  Maybe<uv_file> check = CheckFile(path, LEAVE_OPEN_AFTER_CHECK);
+  if (check.IsNothing()) {
+    auto entry = env->package_json_cache.emplace(path,
+        PackageConfig { Exists::No, IsValid::Yes, HasMain::No, "" });
+    return entry.first->second;
+  }
+
+  Isolate* isolate = env->isolate();
+  v8::HandleScope handle_scope(isolate);
+
+  std::string pkg_src = ReadFile(check.FromJust());
+  uv_fs_t fs_req;
+  CHECK_EQ(0, uv_fs_close(nullptr, &fs_req, check.FromJust(), nullptr));
+  uv_fs_req_cleanup(&fs_req);
+
+  Local<String> src;
+  if (!String::NewFromUtf8(isolate,
+                           pkg_src.c_str(),
+                           v8::NewStringType::kNormal,
+                           pkg_src.length()).ToLocal(&src)) {
+    auto entry = env->package_json_cache.emplace(path,
+        PackageConfig { Exists::No, IsValid::Yes, HasMain::No, "" });
+    return entry.first->second;
+  }
+
+  Local<Value> pkg_json_v;
+  Local<Object> pkg_json;
+
+  if (!JSON::Parse(env->context(), src).ToLocal(&pkg_json_v) ||
+      !pkg_json_v->ToObject(env->context()).ToLocal(&pkg_json)) {
+    auto entry = env->package_json_cache.emplace(path,
+        PackageConfig { Exists::Yes, IsValid::No, HasMain::No, "" });
+    return entry.first->second;
+  }
+
+  Local<Value> pkg_main;
+  HasMain::Bool has_main = HasMain::No;
+  std::string main_std;
+  if (pkg_json->Get(env->context(), env->main_string()).ToLocal(&pkg_main)) {
+    has_main = HasMain::Yes;
+    Utf8Value main_utf8(isolate, pkg_main);
+    main_std.assign(std::string(*main_utf8, main_utf8.length()));
+  }
+
+  auto entry = env->package_json_cache.emplace(path,
+      PackageConfig { Exists::Yes, IsValid::Yes, has_main, "" });
+  return entry.first->second;
+}
+
 enum ResolveExtensionsOptions {
   TRY_EXACT_NAME,
   ONLY_VIA_EXTENSIONS
@@ -502,7 +556,8 @@ enum ResolveExtensionsOptions {
 template<ResolveExtensionsOptions options>
 Maybe<URL> ResolveExtensions(const URL& search) {
   if (options == TRY_EXACT_NAME) {
-    Maybe<uv_file> check = CheckFile(search);
+    std::string filePath = search.ToFilePath();
+    Maybe<uv_file> check = CheckFile(filePath);
     if (!check.IsNothing()) {
       return Just(search);
     }
@@ -510,7 +565,7 @@ Maybe<URL> ResolveExtensions(const URL& search) {
 
   for (const char* extension : EXTENSIONS) {
     URL guess(search.path() + extension, &search);
-    Maybe<uv_file> check = CheckFile(guess);
+    Maybe<uv_file> check = CheckFile(guess.ToFilePath());
     if (!check.IsNothing()) {
       return Just(guess);
     }
@@ -525,44 +580,18 @@ inline Maybe<URL> ResolveIndex(const URL& search) {
 
 Maybe<URL> ResolveMain(Environment* env, const URL& search) {
   URL pkg("package.json", &search);
-  Maybe<uv_file> check = CheckFile(pkg, LEAVE_OPEN_AFTER_CHECK);
+
-  if (check.IsNothing()) {
+  const PackageConfig& pjson =
+      GetPackageConfig(env, pkg.ToFilePath());
+  // Note invalid package.json should throw in resolver
+  // currently we silently ignore which is incorrect
+  if (!pjson.exists || !pjson.is_valid || !pjson.has_main) {
     return Nothing<URL>();
   }
-
+  if (!ShouldBeTreatedAsRelativeOrAbsolutePath(pjson.main)) {
-  Isolate* isolate = env->isolate();
+    return Resolve(env, "./" + pjson.main, search);
-  Local<Context> context = isolate->GetCurrentContext();
-  std::string pkg_src = ReadFile(check.FromJust());
-  uv_fs_t fs_req;
-  uv_fs_close(nullptr, &fs_req, check.FromJust(), nullptr);
-  uv_fs_req_cleanup(&fs_req);
-
-  // It's not okay for the called of this method to not be able to tell
-  // whether an exception is pending or not.
-  TryCatch try_catch(isolate);
-
-  Local<String> src;
-  if (!String::NewFromUtf8(isolate,
-                           pkg_src.c_str(),
-                           v8::NewStringType::kNormal,
-                           pkg_src.length()).ToLocal(&src)) {
-    return Nothing<URL>();
   }
-
+  return Resolve(env, pjson.main, search);
-  Local<Value> pkg_json;
-  if (!JSON::Parse(context, src).ToLocal(&pkg_json) || !pkg_json->IsObject())
-    return Nothing<URL>();
-  Local<Value> pkg_main;
-  if (!pkg_json.As<Object>()->Get(context, env->main_string())
-                              .ToLocal(&pkg_main) || !pkg_main->IsString()) {
-    return Nothing<URL>();
-  }
-  Utf8Value main_utf8(isolate, pkg_main.As<String>());
-  std::string main_std(*main_utf8, main_utf8.length());
-  if (!ShouldBeTreatedAsRelativeOrAbsolutePath(main_std)) {
-    main_std.insert(0, "./");
-  }
-  return Resolve(env, main_std, search);
 }
 
 Maybe<URL> ResolveModule(Environment* env,
@@ -572,7 +601,8 @@ Maybe<URL> ResolveModule(Environment* env,
   URL dir("");
   do {
     dir = parent;
-    Maybe<URL> check = Resolve(env, "./node_modules/" + specifier, dir, true);
+    Maybe<URL> check =
+        Resolve(env, "./node_modules/" + specifier, dir, IgnoreMain);
     if (!check.IsNothing()) {
       const size_t limit = specifier.find('/');
       const size_t spec_len =
@@ -594,8 +624,8 @@ Maybe<URL> ResolveModule(Environment* env,
 
 Maybe<URL> ResolveDirectory(Environment* env,
                             const URL& search,
-                            bool read_pkg_json) {
+                            PackageMainCheck check_pjson_main) {
-  if (read_pkg_json) {
+  if (check_pjson_main) {
     Maybe<URL> main = ResolveMain(env, search);
     if (!main.IsNothing())
       return main;
@@ -605,15 +635,14 @@ Maybe<URL> ResolveDirectory(Environment* env,
 
 }  // anonymous namespace
 
-
 Maybe<URL> Resolve(Environment* env,
                    const std::string& specifier,
                    const URL& base,
-                   bool read_pkg_json) {
+                   PackageMainCheck check_pjson_main) {
   URL pure_url(specifier);
   if (!(pure_url.flags() & URL_FLAGS_FAILED)) {
     // just check existence, without altering
-    Maybe<uv_file> check = CheckFile(pure_url);
+    Maybe<uv_file> check = CheckFile(pure_url.ToFilePath());
     if (check.IsNothing()) {
       return Nothing<URL>();
     }
@@ -630,7 +659,7 @@ Maybe<URL> Resolve(Environment* env,
     if (specifier.back() != '/') {
       resolved = URL(specifier + "/", base);
     }
-    return ResolveDirectory(env, resolved, read_pkg_json);
+    return ResolveDirectory(env, resolved, check_pjson_main);
   } else {
     return ResolveModule(env, specifier, base);
   }
@@ -667,7 +696,7 @@ void ModuleWrap::Resolve(const FunctionCallbackInfo<Value>& args) {
     return;
   }
 
-  Maybe<URL> result = node::loader::Resolve(env, specifier_std, url, true);
+  Maybe<URL> result = node::loader::Resolve(env, specifier_std, url);
   if (result.IsNothing() || (result.FromJust().flags() & URL_FLAGS_FAILED)) {
     std::string msg = "Cannot find module " + specifier_std;
     env->ThrowError(msg.c_str());

src/module_wrap.h

@@ -12,10 +12,15 @@
 namespace node {
 namespace loader {
 
+enum PackageMainCheck : bool {
+    CheckMain = true,
+    IgnoreMain = false
+};
+
 v8::Maybe<url::URL> Resolve(Environment* env,
                             const std::string& specifier,
                             const url::URL& base,
-                            bool read_pkg_json = false);
+                            PackageMainCheck read_pkg_json = CheckMain);
 
 class ModuleWrap : public BaseObject {
  public:

test/fixtures/es-module-loaders/example-loader.mjs

@@ -8,7 +8,10 @@ const builtins = new Set(
 );
 const JS_EXTENSIONS = new Set(['.js', '.mjs']);
 
-export function resolve(specifier, parentModuleURL/*, defaultResolve */) {
+const baseURL = new url.URL('file://');
+baseURL.pathname = process.cwd() + '/';
+
+export function resolve(specifier, parentModuleURL = baseURL /*, defaultResolve */) {
   if (builtins.has(specifier)) {
     return {
       url: specifier,

test/fixtures/es-module-loaders/js-loader.mjs

@@ -3,7 +3,11 @@ const builtins = new Set(
   Object.keys(process.binding('natives')).filter(str =>
     /^(?!(?:internal|node|v8)\/)/.test(str))
 )
-export function resolve (specifier, base) {
+
+const baseURL = new _url.URL('file://');
+baseURL.pathname = process.cwd() + '/';
+
+export function resolve (specifier, base = baseURL) {
   if (builtins.has(specifier)) {
     return {
       url: specifier,

test/fixtures/es-modules/noext

@@ -0,0 +1 @@
+exports.cjs = true;

test/parallel/test-module-main-extension-lookup.js

@@ -5,3 +5,5 @@ const { execFileSync } = require('child_process');
 const node = process.argv[0];
 
 execFileSync(node, ['--experimental-modules', 'test/es-module/test-esm-ok']);
+execFileSync(node, ['--experimental-modules',
+                    'test/fixtures/es-modules/noext']);