esm: only register text format when enabled

lib/internal/modules/esm/assert.js

@@ -3,6 +3,7 @@
 const {
   ArrayPrototypeFilter,
   ArrayPrototypeIncludes,
+  ArrayPrototypePush,
   ObjectKeys,
   ObjectPrototypeHasOwnProperty,
   ObjectValues,
@@ -30,7 +31,6 @@
   'commonjs': kImplicitTypeAttribute,
   'json': 'json',
   'module': kImplicitTypeAttribute,
-  'text': 'text',
   'wasm': kImplicitTypeAttribute, // It's unclear whether the HTML spec will require an type attribute or not for Wasm; see https://github.com/WebAssembly/esm-integration/issues/42
 };
 // NOTE: Don't add bytes support yet as it requires Uint8Arrays backed by immutable ArrayBuffers,
@@ -48,6 +48,15 @@
   ObjectValues(formatTypeMap),
   (type) => type !== kImplicitTypeAttribute);
 
+function initializeImportAttributes() {
+  if (getOptionValue('--experimental-import-text') &&
+    formatTypeMap['text'] === undefined
+  ) {
+    formatTypeMap['text'] = 'text';
+    ArrayPrototypePush(supportedTypeAttributes, 'text');
+  }
+}
+
 /**
  * Test a module's import attributes.
  * @param {string} url The URL of the imported module, for error reporting.
@@ -67,12 +76,6 @@
   }
   const validType = formatTypeMap[format];
 
-  if (validType !== undefined &&
-    importAttributes.type === 'text' &&
-    !getOptionValue('--experimental-import-text')) {
-    throw new ERR_IMPORT_ATTRIBUTE_UNSUPPORTED('type', importAttributes.type, url);
-  }
-
   switch (validType) {
     case undefined:
       // Ignore attributes for module formats we don't recognize, to allow new
@@ -122,6 +125,7 @@
 
 
 module.exports = {
+  initializeImportAttributes,
   kImplicitTypeAttribute,
   validateAttributes,
 };

lib/internal/process/pre_execution.js

@@ -169,6 +169,8 @@ function prepareExecution(options) {
 
   const { initializeExtensionFormatMap } = require('internal/modules/esm/get_format');
   initializeExtensionFormatMap();
+  const { initializeImportAttributes } = require('internal/modules/esm/assert');
+  initializeImportAttributes();
 
   setupVmModules();
   if (initializeModules) {

test/es-module/test-esm-loader-text-format.mjs

@@ -0,0 +1,23 @@
+import '../common/index.mjs';
+import assert from 'node:assert';
+import { registerHooks } from 'node:module';
+
+// A user loader can use `text` with and without import attributes without the feature flag.
+
+registerHooks({
+  load(url, context, nextLoad) {
+    if (url.endsWith('.txt')) {
+      return nextLoad(url, { ...context, format: 'text' });
+    }
+    return nextLoad(url, context);
+  },
+});
+
+const { default: text } = await import('../fixtures/file-to-read-without-bom.txt');
+const { default: empty } = await import(
+  '../fixtures/empty.txt',
+  { with: { type: 'text' } }
+);
+
+assert.strictEqual(text, 'abc\ndef\nghi\n');
+assert.strictEqual(empty, '');