esm: implement import.meta.main

Boolean value to check if a module is run as entry point of the current
process.

Fixes: https://gtihub.com/nodejs/modules/issues/274

doc/api/esm.md

@@ -784,8 +784,10 @@ import _ from 'data:application/json,"world!"';
 * {Object}
 
 The `import.meta` metaproperty is an `Object` that contains the following
-property:
+properties:
 
+* `main` {boolean} `true` when the current module is the entry point of
+  the current process.
 * `url` {string} The absolute `file:` URL of the module.
 
 ## Differences Between ES Modules and CommonJS
@@ -820,6 +822,8 @@ const __filename = fileURLToPath(import.meta.url);
 const __dirname = dirname(__filename);
 ```
 
+Equivalent of `require.main === module` is [`import.meta.main`][].
+
 ### No `require.resolve`
 
 Former use cases relying on `require.resolve` to determine the resolved path
@@ -1694,6 +1698,7 @@ success!
 [`getFormat` hook]: #esm_code_getformat_code_hook
 [`import()`]: #esm_import-expressions
 [`import.meta.url`]: #esm_import_meta
+[`import.meta.main`]: #esm_import_meta
 [`import`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/import
 [`module.createRequire()`]: modules.html#modules_module_createrequire_filename
 [`module.syncBuiltinESMExports()`]: modules.html#modules_module_syncbuiltinesmexports

lib/internal/modules/esm/translators.js

@@ -3,6 +3,8 @@
 /* global WebAssembly */
 
 const {
+  Boolean,
+  FunctionPrototypeBind,
   JSONParse,
   ObjectKeys,
   SafeMap,
@@ -66,11 +68,12 @@ function initializeImportMeta(meta, { url }) {
   // Alphabetical
   if (experimentalImportMetaResolve)
     meta.resolve = createImportMetaResolve(url);
+  meta.main = Boolean(this?.isMain);
   meta.url = url;
 }
 
 // Strategy for loading a standard JavaScript module
-translators.set('module', async function moduleStrategy(url) {
+translators.set('module', async function moduleStrategy(url, isMain) {
   let { source } = await this._getSource(
     url, { format: 'module' }, defaultGetSource);
   source = `${source}`;
@@ -80,7 +83,9 @@ translators.set('module', async function moduleStrategy(url) {
   debug(`Translating StandardModule ${url}`);
   const module = new ModuleWrap(url, undefined, source, 0, 0);
   moduleWrap.callbackMap.set(module, {
-    initializeImportMeta,
+    initializeImportMeta: isMain ?
+      FunctionPrototypeBind(initializeImportMeta, { isMain }) :
+      initializeImportMeta,
     importModuleDynamically,
   });
   return module;

test/es-module/test-esm-import-meta-main.mjs

@@ -0,0 +1,11 @@
+import { mustCall } from '../common/index.mjs';
+import fixtures from '../common/fixtures.js';
+import assert from 'assert';
+
+assert.strictEqual(import.meta.main, true);
+
+import(fixtures.path('/es-modules/import-meta-main.mjs')).then(
+  mustCall(({ isMain }) => {
+    assert.strictEqual(isMain, false);
+  })
+);

test/es-module/test-esm-import-meta.mjs

@@ -3,7 +3,7 @@ import assert from 'assert';
 
 assert.strictEqual(Object.getPrototypeOf(import.meta), null);
 
-const keys = ['url'];
+const keys = ['main', 'url'];
 assert.deepStrictEqual(Reflect.ownKeys(import.meta), keys);
 
 const descriptors = Object.getOwnPropertyDescriptors(import.meta);

test/fixtures/es-modules/import-meta-main.mjs

@@ -0,0 +1 @@
+export const isMain = import.meta.main;