esm: implement import.meta.main

Boolean value to check if an ES Module is the entrypoint of the
current process.

Implements: #57226

Co-authored-by: Antoine du Hamel <duhamelantoine1995@gmail.com>
PR-URL: https://github.com/nodejs/node/pull/57804
Fixes: https://github.com/nodejs/node/issues/57226
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Guy Bedford <guybedford@gmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Marco Ippolito <marcoippolito54@gmail.com>

doc/api/esm.md

@@ -400,6 +400,35 @@ import { readFileSync } from 'node:fs';
 const buffer = readFileSync(new URL('./data.proto', import.meta.url));
 ```
 
+### `import.meta.main`
+
+<!-- YAML
+added:
+  - REPLACEME
+-->
+
+> Stability: 1.0 - Early development
+
+* {boolean} `true` when the current module is the entry point of the current process; `false` otherwise.
+
+Equivalent to `require.main === module` in CommonJS.
+
+Analogous to Python's `__name__ == "__main__"`.
+
+```js
+export function foo() {
+  return 'Hello, world';
+}
+
+function main() {
+  const message = foo();
+  console.log(message);
+}
+
+if (import.meta.main) main();
+// `foo` can be imported from another module without possible side-effects from `main`
+```
+
 ### `import.meta.resolve(specifier)`
 
 <!-- YAML
@@ -616,6 +645,10 @@ These CommonJS variables are not available in ES modules.
 They can instead be loaded with [`module.createRequire()`][] or
 [`process.dlopen`][].
 
+#### No `require.main`
+
+To replace `require.main === module`, there is the [`import.meta.main`][] API.
+
 #### No `require.resolve`
 
 Relative resolution can be handled via `new URL('./local', import.meta.url)`.
@@ -1181,6 +1214,7 @@ resolution for ESM specifiers is [commonjs-extension-resolution-loader][].
 [`import()`]: #import-expressions
 [`import.meta.dirname`]: #importmetadirname
 [`import.meta.filename`]: #importmetafilename
+[`import.meta.main`]: #importmetamain
 [`import.meta.resolve`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/import.meta/resolve
 [`import.meta.url`]: #importmetaurl
 [`import`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/import

lib/internal/main/worker_thread.js

@@ -203,6 +203,20 @@ port.on('message', (message) => {
         break;
       }
 
+      case 'data-url': {
+        const { runEntryPointWithESMLoader } = require('internal/modules/run_main');
+
+        RegExpPrototypeExec(/^/, ''); // Necessary to reset RegExp statics before user code runs.
+        const promise = runEntryPointWithESMLoader((cascadedLoader) => {
+          return cascadedLoader.import(filename, undefined, { __proto__: null }, undefined, true);
+        });
+
+        PromisePrototypeThen(promise, undefined, (e) => {
+          workerOnGlobalUncaughtException(e, true);
+        });
+        break;
+      }
+
       default: {
         // script filename
         // runMain here might be monkey-patched by users in --require.

lib/internal/modules/esm/initialize_import_meta.js

@@ -51,12 +51,12 @@ function createImportMetaResolve(defaultParentURL, loader, allowParentURL) {
 /**
  * Create the `import.meta` object for a module.
  * @param {object} meta
- * @param {{url: string}} context
+ * @param {{url: string, isMain?: boolean}} context
  * @param {typeof import('./loader.js').ModuleLoader} loader Reference to the current module loader
  * @returns {{dirname?: string, filename?: string, url: string, resolve?: Function}}
  */
 function initializeImportMeta(meta, context, loader) {
-  const { url } = context;
+  const { url, isMain } = context;
 
   // Alphabetical
   if (StringPrototypeStartsWith(url, 'file:') === true) {
@@ -65,6 +65,8 @@ function initializeImportMeta(meta, context, loader) {
     setLazyPathHelpers(meta, url);
   }
 
+  meta.main = !!isMain;
+
   if (!loader || loader.allowImportMetaResolve) {
     meta.resolve = createImportMetaResolve(url, loader, experimentalImportMetaResolve);
   }

lib/internal/modules/esm/loader.js

@@ -248,10 +248,11 @@ class ModuleLoader {
    *
    * @param {string} source Source code of the module.
    * @param {string} url URL of the module.
+   * @param {{ isMain?: boolean }|undefined} context - context object containing module metadata.
    * @returns {object} The module wrap object.
    */
-  createModuleWrap(source, url) {
-    return compileSourceTextModule(url, source, this);
+  createModuleWrap(source, url, context = kEmptyObject) {
+    return compileSourceTextModule(url, source, this, context);
   }
 
   /**
@@ -289,7 +290,8 @@ class ModuleLoader {
    * @returns {Promise<object>} The module object.
    */
   eval(source, url, isEntryPoint = false) {
-    const wrap = this.createModuleWrap(source, url);
+    const context = isEntryPoint ? { isMain: true } : undefined;
+    const wrap = this.createModuleWrap(source, url, context);
     return this.executeModuleJob(url, wrap, isEntryPoint);
   }
 

lib/internal/modules/esm/translators.js

@@ -103,7 +103,8 @@ translators.set('module', function moduleStrategy(url, source, isMain) {
   source = stringify(source);
   debug(`Translating StandardModule ${url}`);
   const { compileSourceTextModule } = require('internal/modules/esm/utils');
-  const module = compileSourceTextModule(url, source, this);
+  const context = isMain ? { isMain } : undefined;
+  const module = compileSourceTextModule(url, source, this, context);
   return module;
 });
 

lib/internal/modules/esm/utils.js

@@ -42,6 +42,7 @@ const {
 const {
   emitExperimentalWarning,
   getCWDURL,
+  kEmptyObject,
 } = require('internal/util');
 const assert = require('internal/assert');
 const {
@@ -188,7 +189,7 @@ function registerModule(referrer, registry) {
  */
 function defaultInitializeImportMetaForModule(meta, wrap) {
   const cascadedLoader = require('internal/modules/esm/loader').getOrInitializeCascadedLoader();
-  return cascadedLoader.importMetaInitialize(meta, { url: wrap.url });
+  return cascadedLoader.importMetaInitialize(meta, { url: wrap.url, isMain: wrap.isMain });
 }
 
 /**
@@ -342,15 +343,22 @@ async function initializeHooks() {
  * @param {string} source Source code of the module.
  * @param {typeof import('./loader.js').ModuleLoader|undefined} cascadedLoader If provided,
  *        register the module for default handling.
+ * @param {{ isMain?: boolean }|undefined} context - context object containing module metadata.
  * @returns {ModuleWrap}
  */
-function compileSourceTextModule(url, source, cascadedLoader) {
+function compileSourceTextModule(url, source, cascadedLoader, context = kEmptyObject) {
   const hostDefinedOption = cascadedLoader ? source_text_module_default_hdo : undefined;
   const wrap = new ModuleWrap(url, undefined, source, 0, 0, hostDefinedOption);
 
   if (!cascadedLoader) {
     return wrap;
   }
+
+  const { isMain } = context;
+  if (isMain) {
+    wrap.isMain = true;
+  }
+
   // Cache the source map for the module if present.
   if (wrap.sourceMapURL) {
     maybeCacheSourceMap(url, source, wrap, false, undefined, wrap.sourceMapURL);

lib/internal/worker.js

@@ -7,7 +7,6 @@ const {
   AtomicsAdd,
   Float64Array,
   FunctionPrototypeBind,
-  JSONStringify,
   MathMax,
   ObjectEntries,
   Promise,
@@ -167,8 +166,8 @@ class Worker extends EventEmitter {
       doEval = 'classic';
     } else if (isURL(filename) && filename.protocol === 'data:') {
       url = null;
-      doEval = 'module';
-      filename = `import ${JSONStringify(`${filename}`)}`;
+      doEval = 'data-url';
+      filename = `${filename}`;
     } else {
       doEval = false;
       if (isURL(filename)) {

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

@@ -0,0 +1,74 @@
+import { spawnPromisified } from '../common/index.mjs';
+import * as fixtures from '../common/fixtures.js';
+import assert from 'node:assert/strict';
+import { describe, it } from 'node:test';
+
+const importMetaMainScript = `
+import assert from 'node:assert/strict';
+
+assert.strictEqual(import.meta.main, true, 'import.meta.main should evaluate true in main module');
+
+const { isMain: importedModuleIsMain } = await import(
+    ${JSON.stringify(fixtures.fileURL('es-modules/import-meta-main.mjs'))}
+);
+assert.strictEqual(importedModuleIsMain, false, 'import.meta.main should evaluate false in imported module');
+`;
+
+function wrapScriptInEvalWorker(script) {
+  return `
+  import { Worker } from 'node:worker_threads';
+  new Worker(${JSON.stringify(script)}, { eval: true });
+  `;
+}
+
+function convertScriptSourceToDataUrl(script) {
+  return new URL(`data:text/javascript,${encodeURIComponent(script)}`);
+}
+
+function wrapScriptInUrlWorker(script) {
+  return `
+  import { Worker } from 'node:worker_threads';
+  new Worker(new URL(${JSON.stringify(convertScriptSourceToDataUrl(script))}));
+  `;
+}
+
+describe('import.meta.main in evaluated scripts', () => {
+  it('should evaluate true in evaluated script', async () => {
+    const result = await spawnPromisified(
+      process.execPath,
+      ['--input-type=module', '--eval', importMetaMainScript],
+    );
+    assert.deepStrictEqual(result, {
+      stderr: '',
+      stdout: '',
+      code: 0,
+      signal: null,
+    });
+  });
+
+  it('should evaluate true in worker instantiated with module source by evaluated script', async () => {
+    const result = await spawnPromisified(
+      process.execPath,
+      ['--input-type=module', '--eval', wrapScriptInEvalWorker(importMetaMainScript)],
+    );
+    assert.deepStrictEqual(result, {
+      stderr: '',
+      stdout: '',
+      code: 0,
+      signal: null,
+    });
+  });
+
+  it('should evaluate true in worker instantiated with `data:` URL by evaluated script', async () => {
+    const result = await spawnPromisified(
+      process.execPath,
+      ['--input-type=module', '--eval', wrapScriptInUrlWorker(importMetaMainScript)],
+    );
+    assert.deepStrictEqual(result, {
+      stderr: '',
+      stdout: '',
+      code: 0,
+      signal: null,
+    });
+  });
+});

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

@@ -0,0 +1,26 @@
+import '../common/index.mjs';
+import assert from 'node:assert/strict';
+import { Worker } from 'node:worker_threads';
+
+function get_environment() {
+  if (process.env.HAS_STARTED_WORKER) return 'in worker thread started by ES Module';
+  return 'in ES Module';
+}
+
+assert.strictEqual(
+  import.meta.main,
+  true,
+  `\`import.meta.main\` at top-level module ${get_environment()} should evaluate \`true\``
+);
+
+const { isMain: importedModuleIsMain } = await import('../fixtures/es-modules/import-meta-main.mjs');
+assert.strictEqual(
+  importedModuleIsMain,
+  false,
+  `\`import.meta.main\` at dynamically imported module ${get_environment()} should evaluate \`false\``
+);
+
+if (!process.env.HAS_STARTED_WORKER) {
+  process.env.HAS_STARTED_WORKER = 1;
+  new Worker(import.meta.filename);
+}

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 = ['dirname', 'filename', 'resolve', 'url'];
+const keys = ['dirname', 'filename', 'main', 'resolve', '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;