[WIP] Support requiring .mjs files

lib/internal/modules/cjs/loader.js

@@ -86,6 +86,7 @@ const {
   ERR_REQUIRE_ESM
 } = require('internal/errors').codes;
 const { validateString } = require('internal/validators');
+const { promiseWait } = internalBinding('task_queue');
 const pendingDeprecation = getOptionValue('--pending-deprecation');
 
 module.exports = {
@@ -1038,36 +1039,34 @@ Module.prototype.load = function(filename) {
   this.paths = Module._nodeModulePaths(path.dirname(filename));
 
   const extension = findLongestRegisteredExtension(filename);
-  // allow .mjs to be overridden
-  if (filename.endsWith('.mjs') && !Module._extensions['.mjs']) {
-    throw new ERR_REQUIRE_ESM(filename);
-  }
   Module._extensions[extension](this, filename);
   this.loaded = true;
 
-  const ESMLoader = asyncESM.ESMLoader;
-  const url = `${pathToFileURL(filename)}`;
-  const module = ESMLoader.moduleMap.get(url);
-  // Create module entry at load time to snapshot exports correctly
-  const exports = this.exports;
-  // Called from cjs translator
-  if (module !== undefined && module.module !== undefined) {
-    if (module.module.getStatus() >= kInstantiated)
-      module.module.setExport('default', exports);
-  } else {
-    // Preemptively cache
-    // We use a function to defer promise creation for async hooks.
-    ESMLoader.moduleMap.set(
-      url,
-      // Module job creation will start promises.
-      // We make it a function to lazily trigger those promises
-      // for async hooks compatibility.
-      () => new ModuleJob(ESMLoader, url, () =>
-        new ModuleWrap(url, undefined, ['default'], function() {
-          this.setExport('default', exports);
-        })
-      , false /* isMain */, false /* inspectBrk */)
-    );
+  if (extension !== '.mjs') {
+    const ESMLoader = asyncESM.ESMLoader;
+    const url = `${pathToFileURL(filename)}`;
+    const module = ESMLoader.moduleMap.get(url);
+    // Create module entry at load time to snapshot exports correctly
+    const exports = this.exports;
+    // Called from cjs translator
+    if (module !== undefined && module.module !== undefined) {
+      if (module.module.getStatus() >= kInstantiated)
+        module.module.setExport('default', exports);
+    } else {
+      // Preemptively cache
+      // We use a function to defer promise creation for async hooks.
+      ESMLoader.moduleMap.set(
+        url,
+        // Module job creation will start promises.
+        // We make it a function to lazily trigger those promises
+        // for async hooks compatibility.
+        () => new ModuleJob(ESMLoader, url, () =>
+          new ModuleWrap(url, undefined, ['default'], function() {
+            this.setExport('default', exports);
+          })
+        , false /* isMain */, false /* inspectBrk */)
+      );
+    }
   }
 };
 
@@ -1246,6 +1245,19 @@ Module._extensions['.node'] = function(module, filename) {
   return process.dlopen(module, path.toNamespacedPath(filename));
 };
 
+Module._extensions['.mjs'] = function(module, filename) {
+  const ESMLoader = asyncESM.ESMLoader;
+  const url = `${pathToFileURL(filename)}`;
+  const job = ESMLoader.getModuleJobWorker(url, 'module');
+  /* So long as builtin esm resolve is sync, this will complete sync
+   * (if the loader is extensible, an async loader will be have an observable
+   * effect here)
+   */
+  const instantiated = promiseWait(job.instantiate());
+  module.exports = instantiated.getNamespace();
+  instantiated.evaluate(-1, false);
+}
+
 function createRequireFromPath(filename) {
   // Allow a directory to be passed as the filename
   const trailingSlash =

lib/internal/modules/esm/loader.js

@@ -152,8 +152,7 @@ class Loader {
     }
   }
 
-  async getModuleJob(specifier, parentURL) {
-    const { url, format } = await this.resolve(specifier, parentURL);
+  getModuleJobWorker(url, format, parentURL) {
     let job = this.moduleMap.get(url);
     // CommonJS will set functions for lazy job evaluation.
     if (typeof job === 'function')
@@ -188,6 +187,11 @@ class Loader {
     this.moduleMap.set(url, job);
     return job;
   }
+
+  async getModuleJob(specifier, parentURL) {
+    const { url, format } = await this.resolve(specifier, parentURL);
+    return this.getModuleJobWorker(url, format, parentURL);
+  }
 }
 
 ObjectSetPrototypeOf(Loader.prototype, null);

src/node_task_queue.cc

@@ -119,6 +119,45 @@ static void SetPromiseRejectCallback(
   env->set_promise_reject_callback(args[0].As<Function>());
 }
 
+/**
+ * Immediately unwraps a promise into a return value or throw, if possible
+ * If not, runs the event loop and microtask queue until it is unwrapable. 
+ */
+static void PromiseWait(const FunctionCallbackInfo<Value>& args) {
+  if (!args[0]->IsPromise()) {
+    args.GetReturnValue().Set(args[0]);
+    return;
+  }
+  v8::Local<v8::Promise> promise = args[0].As<v8::Promise>();
+  if (promise->State() == v8::Promise::kFulfilled) {
+    args.GetReturnValue().Set(promise->Result());
+    return;
+  }
+  Isolate* isolate = args.GetIsolate();
+  if (promise->State() == v8::Promise::kRejected) {
+    isolate->ThrowException(promise->Result());
+    return;
+  }
+
+  Environment* env = Environment::GetCurrent(args);
+
+  uv_loop_t* loop = env->event_loop();
+  int state = promise->State();
+  while (state == v8::Promise::kPending) {
+    isolate->RunMicrotasks();
+    if (uv_loop_alive(loop) && promise->State() == v8::Promise::kPending) {
+      uv_run(loop, UV_RUN_ONCE);
+    }
+    state = promise->State();
+  }
+
+  if (promise->State() == v8::Promise::kRejected) {
+    isolate->ThrowException(promise->Result());
+    return;
+  }
+  args.GetReturnValue().Set(promise->Result());
+}
+
 static void Initialize(Local<Object> target,
                        Local<Value> unused,
                        Local<Context> context,
@@ -145,6 +184,8 @@ static void Initialize(Local<Object> target,
   env->SetMethod(target,
                  "setPromiseRejectCallback",
                  SetPromiseRejectCallback);
+
+  env->SetMethod(target, "promiseWait", PromiseWait);
 }
 
 }  // namespace task_queue

test/parallel/test-require-mjs.js

@@ -2,10 +2,4 @@
 require('../common');
 const assert = require('assert');
 
-assert.throws(
-  () => require('../fixtures/es-modules/test-esm-ok.mjs'),
-  {
-    message: /Must use import to load ES Module/,
-    code: 'ERR_REQUIRE_ESM'
-  }
-);
+assert.equal(require('../fixtures/es-modules/test-esm-ok.mjs').default, true);