module: expose module format by module loader

Module loaders may change a requested module format. It is not safe to
determine a `.js` format by `package.json#type` as well. Expose a helper
function to frameworks that load js files to avoid relying on
`import`/`require` failover-and-retry.

Author: legendecas

doc/api/module.md

@@ -1556,6 +1556,56 @@ Running `node --import 'data:text/javascript,import { register } from "node:modu
 or `node --import ./import-map-sync-hooks.js main.js`
 should print `some module!`.
 
+## Module Hooks Reflection
+
+<!-- YAML
+added: REPLACEME
+-->
+
+> Stability: 1.1 - Active development
+
+### `module.loadModule(specifier, parentURL[, options])`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+* `specifier` {string} The URL of the module to load.
+* `parentURL` {string} The module importing this one.
+* `options` {Object} Optional
+  * `importAttributes` {Object} An object whose key-value pairs represent the
+    attributes for the module to import.
+* Returns: {Object}
+  * `format` {string} The resolved format of the module.
+  * `source` {string|ArrayBuffer|TypedArray|null} The source for Node.js to evaluate.
+  * `url` {string} The resolved URL of the module.
+
+Request to load a module using the current module hooks. This does not
+evaluate the module, it only returns the resolved URL and source code.
+This is useful for determining the format of a module and its source code.
+
+This is the recommended way to detect a module format rather than referring
+to the `package.json` file or the file extension. The `format` property
+is one of the values listed in the [`module.kModuleFormats`][].
+
+### `module.kModuleFormats`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+* Returns: {Object} An object with the following properties:
+  * `addon` {string} Only present when the `--experimental-addon-modules` flag is enabled.
+  * `builtin` {string}
+  * `commonjs` {string}
+  * `json` {string}
+  * `module` {string}
+  * `wasm` {string} Only present when the `--experimental-wasm-modules` flag is
+    enabled.
+
+The `kModuleFormats` property is an object that enumerates the module formats
+supported as a final format returned by the `load` hook.
+
 ## Source map v3 support
 
 <!-- YAML
@@ -1778,6 +1828,7 @@ returned object contains the following keys:
 [`module.enableCompileCache()`]: #moduleenablecompilecachecachedir
 [`module.flushCompileCache()`]: #moduleflushcompilecache
 [`module.getCompileCacheDir()`]: #modulegetcompilecachedir
+[`module.kModuleFormats`]: #modulekmoduleformats
 [`module`]: #the-module-object
 [`os.tmpdir()`]: os.md#ostmpdir
 [`registerHooks`]: #moduleregisterhooksoptions

lib/internal/modules/esm/formats.js

@@ -1,6 +1,7 @@
 'use strict';
 
 const {
+  ObjectFreeze,
   RegExpPrototypeExec,
 } = primordials;
 
@@ -20,12 +21,26 @@ const extensionFormatMap = {
   '.mjs': 'module',
 };
 
+/**
+ * @type {Record<string, string>}
+ * Allowed final module formats returned in a load hook of a module loader.
+ */
+const kModuleFormats = {
+  __proto__: null,
+  commonjs: 'commonjs',
+  module: 'module',
+  json: 'json',
+  builtin: 'builtin',
+};
+
 if (experimentalWasmModules) {
   extensionFormatMap['.wasm'] = 'wasm';
+  kModuleFormats.wasm = 'wasm';
 }
 
 if (experimentalAddonModules) {
   extensionFormatMap['.node'] = 'addon';
+  kModuleFormats.addon = 'addon';
 }
 
 if (getOptionValue('--experimental-strip-types')) {
@@ -34,6 +49,9 @@ if (getOptionValue('--experimental-strip-types')) {
   extensionFormatMap['.cts'] = 'commonjs-typescript';
 }
 
+ObjectFreeze(extensionFormatMap);
+ObjectFreeze(kModuleFormats);
+
 /**
  * @param {string} mime
  * @returns {string | null}
@@ -68,6 +86,7 @@ function getFormatOfExtensionlessFile(url) {
 }
 
 module.exports = {
+  kModuleFormats,
   extensionFormatMap,
   getFormatOfExtensionlessFile,
   mimeToFormat,

lib/internal/modules/esm/loader.js

@@ -58,6 +58,7 @@ const {
 let defaultResolve, defaultLoad, defaultLoadSync, importMetaInitializer;
 
 const { tracingChannel } = require('diagnostics_channel');
+const { validateObject } = require('internal/validators');
 const onImport = tracingChannel('module.import');
 
 let debug = require('internal/util/debuglog').debuglog('esm', (fn) => {
@@ -1061,9 +1062,40 @@ function register(specifier, parentURL = undefined, options) {
   );
 }
 
+async function loadModule(specifier, parentURL, options = kEmptyObject) {
+  specifier = `${specifier}`;
+  parentURL = `${parentURL}`;
+
+  validateObject(options, 'options');
+  const { importAttributes = { __proto__: null } } = options;
+  validateObject(importAttributes, 'options.importAttributes');
+
+  const loader = getOrInitializeCascadedLoader();
+  const { url, format: resolvedFormat } = await loader.resolve(specifier, parentURL, importAttributes);
+  const result = await loader.load(url, { format: resolvedFormat, importAttributes });
+  const { source } = result;
+  let { format: finalFormat } = result;
+
+  // Translate internal formats to public ones.
+  if (finalFormat === 'commonjs-typescript') {
+    finalFormat = 'commonjs';
+  }
+  if (finalFormat === 'module-typescript') {
+    finalFormat = 'module';
+  }
+
+  return {
+    __proto__: null,
+    url,
+    format: finalFormat,
+    source,
+  };
+}
+
 module.exports = {
   createModuleLoader,
   getHooksProxy,
   getOrInitializeCascadedLoader,
   register,
+  loadModule,
 };

lib/module.js

@@ -6,7 +6,7 @@ const {
   setSourceMapsSupport,
 } = require('internal/source_map/source_map_cache');
 const { Module } = require('internal/modules/cjs/loader');
-const { register } = require('internal/modules/esm/loader');
+const { register, loadModule } = require('internal/modules/esm/loader');
 const {
   SourceMap,
 } = require('internal/source_map/source_map');
@@ -20,6 +20,7 @@ const {
   findPackageJSON,
 } = require('internal/modules/package_json_reader');
 const { stripTypeScriptTypes } = require('internal/modules/typescript');
+const { kModuleFormats } = require('internal/modules/esm/formats');
 
 Module.register = register;
 Module.constants = constants;
@@ -29,6 +30,10 @@ Module.flushCompileCache = flushCompileCache;
 Module.getCompileCacheDir = getCompileCacheDir;
 Module.stripTypeScriptTypes = stripTypeScriptTypes;
 
+// Module reflection APIs
+Module.loadModule = loadModule;
+Module.kModuleFormats = kModuleFormats;
+
 // SourceMap APIs
 Module.findSourceMap = findSourceMap;
 Module.SourceMap = SourceMap;

test/fixtures/typescript/legacy-module/README.md

@@ -0,0 +1,5 @@
+# Legacy TypeScript Module
+
+When `tsconfig.json` is set to `module: "node16"` or any `node*`, the TypeScript compiler will
+produce the output in the format by the extension (e.g. `.cts` or `.mts`), or set by the
+`package.json#type` field, regardless of the syntax of the original source code.

test/fixtures/typescript/legacy-module/package.json

@@ -0,0 +1,3 @@
+{
+  "type": "commonjs"
+}

test/fixtures/typescript/legacy-module/test-module-export.cts

@@ -0,0 +1 @@
+export const foo: string = 'Hello, TypeScript!';

test/fixtures/typescript/legacy-module/test-module-export.mts

@@ -0,0 +1 @@
+export const foo: string = 'Hello, TypeScript!';

test/fixtures/typescript/legacy-module/test-module-export.ts

@@ -0,0 +1 @@
+export const foo: string = 'Hello, TypeScript!';

test/parallel/test-module-loadmodule-no-typescript.js

@@ -0,0 +1,24 @@
+// Flags: --no-experimental-strip-types
+
+'use strict';
+
+require('../common');
+const test = require('node:test');
+const assert = require('node:assert');
+const { pathToFileURL } = require('node:url');
+const fixtures = require('../common/fixtures');
+const { loadModule } = require('node:module');
+
+const parentURL = pathToFileURL(__filename);
+
+test('should reject a TypeScript module', async () => {
+  const fileUrl = fixtures.fileURL('typescript/legacy-module/test-module-export.ts');
+  await assert.rejects(
+    async () => {
+      await loadModule(fileUrl, parentURL);
+    },
+    {
+      code: 'ERR_UNKNOWN_FILE_EXTENSION',
+    }
+  );
+});

test/parallel/test-module-loadmodule-typescript.js

@@ -0,0 +1,49 @@
+// Flags: --experimental-strip-types
+
+'use strict';
+
+require('../common');
+const test = require('node:test');
+const assert = require('node:assert');
+const { pathToFileURL } = require('node:url');
+const fixtures = require('../common/fixtures');
+const { loadModule, kModuleFormats } = require('node:module');
+
+const parentURL = pathToFileURL(__filename);
+
+test('should load a TypeScript module source by package.json type', async () => {
+  // Even if the .ts file contains module syntax, it should be loaded as a CommonJS module
+  // because the package.json type is set to "commonjs".
+
+  const fileUrl = fixtures.fileURL('typescript/legacy-module/test-module-export.ts');
+  const { url, format, source } = await loadModule(fileUrl, parentURL);
+  assert.strictEqual(format, kModuleFormats.commonjs);
+  assert.strictEqual(url, fileUrl.href);
+
+  // Built-in TypeScript loader loads the source.
+  assert.ok(Buffer.isBuffer(source));
+});
+
+test('should load a TypeScript cts module source by extension', async () => {
+  // By extension, .cts files should be loaded as CommonJS modules.
+
+  const fileUrl = fixtures.fileURL('typescript/legacy-module/test-module-export.cts');
+  const { url, format, source } = await loadModule(fileUrl, parentURL);
+  assert.strictEqual(format, kModuleFormats.commonjs);
+  assert.strictEqual(url, fileUrl.href);
+
+  // Built-in TypeScript loader loads the source.
+  assert.ok(Buffer.isBuffer(source));
+});
+
+test('should load a TypeScript mts module source by extension', async () => {
+  // By extension, .mts files should be loaded as ES modules.
+
+  const fileUrl = fixtures.fileURL('typescript/legacy-module/test-module-export.mts');
+  const { url, format, source } = await loadModule(fileUrl, parentURL);
+  assert.strictEqual(format, kModuleFormats.module);
+  assert.strictEqual(url, fileUrl.href);
+
+  // Built-in TypeScript loader loads the source.
+  assert.ok(Buffer.isBuffer(source));
+});

test/parallel/test-module-loadmodule.js

@@ -0,0 +1,36 @@
+'use strict';
+
+require('../common');
+const test = require('node:test');
+const assert = require('node:assert');
+const { pathToFileURL } = require('node:url');
+const fixtures = require('../common/fixtures');
+const { loadModule, kModuleFormats } = require('node:module');
+
+const parentURL = pathToFileURL(__filename);
+
+test('kModuleFormats is a frozen object', () => {
+  assert.ok(typeof kModuleFormats === 'object');
+  assert.ok(Object.isFrozen(kModuleFormats));
+});
+
+test('should throw if the module is not found', async () => {
+  await assert.rejects(
+    async () => {
+      await loadModule('nonexistent-module', parentURL);
+    },
+    {
+      code: 'ERR_MODULE_NOT_FOUND',
+    }
+  );
+});
+
+test('should load a module', async () => {
+  const fileUrl = fixtures.fileURL('es-modules/cjs.js');
+  const { url, format, source } = await loadModule(fileUrl, parentURL);
+  assert.strictEqual(format, kModuleFormats.commonjs);
+  assert.strictEqual(url, fileUrl.href);
+
+  // source is null and the final builtin loader will read the file.
+  assert.deepStrictEqual(source, null);
+});