esm: remove CLI flag limitation to programmatic registration

lib/internal/errors.js

@@ -1036,11 +1036,6 @@ E('ERR_ENCODING_INVALID_ENCODED_DATA', function(encoding, ret) {
 }, TypeError);
 E('ERR_ENCODING_NOT_SUPPORTED', 'The "%s" encoding is not supported',
   RangeError);
-E('ERR_ESM_LOADER_REGISTRATION_UNAVAILABLE', 'Programmatically registering custom ESM loaders ' +
-  'currently requires at least one custom loader to have been registered via the --experimental-loader ' +
-  'flag. A no-op loader registered via CLI is sufficient (for example: `--experimental-loader ' +
-  '"data:text/javascript,"` with the necessary trailing comma). A future version of Node.js ' +
-  'will remove this requirement.', Error);
 E('ERR_EVAL_ESM_CANNOT_PRINT', '--print cannot be used with ESM input', Error);
 E('ERR_EVENT_RECURSION', 'The event "%s" is already being dispatched', Error);
 E('ERR_FALSY_VALUE_REJECTION', function(reason) {

lib/internal/modules/esm/loader.js

@@ -11,16 +11,18 @@ const {
 } = primordials;
 
 const {
-  ERR_ESM_LOADER_REGISTRATION_UNAVAILABLE,
   ERR_UNKNOWN_MODULE_FORMAT,
 } = require('internal/errors').codes;
 const { getOptionValue } = require('internal/options');
 const { pathToFileURL } = require('internal/url');
-const { emitExperimentalWarning } = require('internal/util');
+const { emitExperimentalWarning, kEmptyObject } = require('internal/util');
 const {
   getDefaultConditions,
 } = require('internal/modules/esm/utils');
 let defaultResolve, defaultLoad, importMetaInitializer;
+let debug = require('internal/util/debuglog').debuglog('esm', (fn) => {
+  debug = fn;
+});
 
 function newModuleMap() {
   const ModuleMap = require('internal/modules/esm/module_map');
@@ -106,15 +108,15 @@ class DefaultModuleLoader {
       setCallbackForWrap(module, {
         initializeImportMeta: (meta, wrap) => this.importMetaInitialize(meta, { url }),
         importModuleDynamically: (specifier, { url }, importAssertions) => {
+          debug('importModuleDynamically: %o', { specifier, url, Loader: this.constructor.name });
           return this.import(specifier, url, importAssertions);
         },
       });
 
       return module;
     };
     const ModuleJob = require('internal/modules/esm/module_job');
-    const job = new ModuleJob(
-      this, url, undefined, evalInstance, false, false);
+    const job = new ModuleJob(this, url, undefined, evalInstance, false, false);
     this.moduleMap.set(url, undefined, job);
     const { module } = await job.run();
 
@@ -285,10 +287,21 @@ class CustomizedModuleLoader extends DefaultModuleLoader {
   /**
    * Instantiate a module loader that uses user-provided custom loader hooks.
    */
-  constructor() {
+  constructor({
+    cjsCache,
+    evalIndex,
+    moduleMap,
+  } = kEmptyObject) {
     super();
 
-    getHooksProxy();
+    if (cjsCache != null) { this.cjsCache = cjsCache; }
+    if (evalIndex != null) { this.evalIndex = evalIndex; }
+    if (moduleMap != null) { this.moduleMap = moduleMap; }
+
+    if (!hooksProxy) {
+      const { HooksProxy } = require('internal/modules/esm/hooks');
+      hooksProxy = new HooksProxy();
+    }
   }
 
   /**
@@ -357,40 +370,29 @@ let emittedExperimentalWarning = false;
  * 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, though separate instances of this
  * class might be instantiated as part of bootstrap for other purposes.
- * @param {boolean} useCustomLoadersIfPresent If the user has provided loaders via the --loader flag, use them.
+ * @param {boolean} forceCustomizedLoaderInMain Ignore whether custom loader(s) have been provided
+ * via CLI and instantiate a CustomizedModuleLoader instance regardless.
  * @returns {DefaultModuleLoader | CustomizedModuleLoader}
  */
-function createModuleLoader(useCustomLoadersIfPresent = true) {
-  if (useCustomLoadersIfPresent &&
-      // Don't spawn a new worker if we're already in a worker thread created by instantiating CustomizedModuleLoader;
-      // doing so would cause an infinite loop.
-      !require('internal/modules/esm/utils').isLoaderWorker()) {
-    const userLoaderPaths = getOptionValue('--experimental-loader');
-    if (userLoaderPaths.length > 0) {
+function createModuleLoader(customizationSetup) {
+  // Don't spawn a new worker if we're already in a worker thread (doing so would cause an infinite loop).
+  if (!require('internal/modules/esm/utils').isLoaderWorker()) {
+    if (
+      customizationSetup ||
+      getOptionValue('--experimental-loader').length > 0
+    ) {
       if (!emittedExperimentalWarning) {
         emitExperimentalWarning('Custom ESM Loaders');
         emittedExperimentalWarning = true;
       }
-      return new CustomizedModuleLoader();
+      debug('instantiating CustomizedModuleLoader');
+      return new CustomizedModuleLoader(customizationSetup);
     }
   }
 
   return new DefaultModuleLoader();
 }
 
-/**
- * Get the HooksProxy instance. If it is not defined, then create a new one.
- * @returns {HooksProxy}
- */
-function getHooksProxy() {
-  if (!hooksProxy) {
-    const { HooksProxy } = require('internal/modules/esm/hooks');
-    hooksProxy = new HooksProxy();
-  }
-
-  return hooksProxy;
-}
-
 /**
  * Register a single loader programmatically.
  * @param {string} specifier
@@ -405,19 +407,19 @@ function getHooksProxy() {
  * ```
  */
 function register(specifier, parentURL = 'data:') {
-  // TODO: Remove this limitation in a follow-up before `register` is released publicly
-  if (getOptionValue('--experimental-loader').length < 1) {
-    throw new ERR_ESM_LOADER_REGISTRATION_UNAVAILABLE();
-  }
+  let moduleLoader = require('internal/process/esm_loader').esmLoader;
 
-  const moduleLoader = require('internal/process/esm_loader').esmLoader;
+  if (!(moduleLoader instanceof CustomizedModuleLoader)) {
+    debug('register called on DefaultModuleLoader; switching to CustomizedModuleLoader');
+    moduleLoader = createModuleLoader(moduleLoader);
+    require('internal/process/esm_loader').esmLoader = moduleLoader;
+  }
 
   moduleLoader.register(`${specifier}`, parentURL);
 }
 
 module.exports = {
   DefaultModuleLoader,
   createModuleLoader,
-  getHooksProxy,
   register,
 };

lib/internal/process/esm_loader.js

@@ -11,15 +11,21 @@ const {
 } = require('internal/process/execution');
 const { pathToFileURL } = require('internal/url');
 const { kEmptyObject } = require('internal/util');
+let debug = require('internal/util/debuglog').debuglog('esm', (fn) => {
+  debug = fn;
+});
 
 let esmLoader;
 
 module.exports = {
   get esmLoader() {
-    return esmLoader ??= createModuleLoader(true);
+    return esmLoader ??= createModuleLoader();
+  },
+  set esmLoader(supplantingLoader) {
+    if (supplantingLoader) esmLoader = supplantingLoader;
   },
   async loadESM(callback) {
-    esmLoader ??= createModuleLoader(true);
+    esmLoader ??= createModuleLoader();
     try {
       const userImports = getOptionValue('--import');
       if (userImports.length > 0) {

test/es-module/test-esm-loader-programmatically.mjs

@@ -189,19 +189,26 @@ describe('ESM: programmatically register loaders', { concurrency: true }, () =>
     assert.strictEqual(lines[3], '');
   });
 
-  it('does not work without dummy CLI loader', async () => {
-    const { code, signal, stdout, stderr } = await spawnPromisified(execPath, [
-      '--input-type=module',
-      '--eval',
-      "import { register } from 'node:module';" +
-      commonEvals.register(fixtures.fileURL('es-module-loaders', 'loader-load-passthru.mjs')) +
-      commonEvals.dynamicImport('console.log("Hello from dynamic import");'),
-    ]);
+  it('works without a CLI flag', async () => {
+      const { code, signal, stdout, stderr } = await spawnPromisified(execPath, [
+        '--no-warnings',
+        '--input-type=module',
+        '--eval',
+        "import { register } from 'node:module';" +
+        commonEvals.register(fixtures.fileURL('es-module-loaders', 'loader-load-passthru.mjs')) +
+        commonEvals.dynamicImport('console.log("Hello from dynamic import");'),
+      ]);
 
-    assert.strictEqual(stdout, '');
-    assert.strictEqual(code, 1);
-    assert.strictEqual(signal, null);
-    assert.match(stderr, /ERR_ESM_LOADER_REGISTRATION_UNAVAILABLE/);
+      assert.strictEqual(stderr, '');
+      assert.strictEqual(code, 0);
+      assert.strictEqual(signal, null);
+
+      const lines = stdout.split('\n');
+
+      assert.match(lines[0], /load passthru/);
+      assert.match(lines[1], /Hello from dynamic import/);
+
+      assert.strictEqual(lines[2], '');
   });
 
   it('does not work with a loader specifier that does not exist', async () => {