module: unflag import assertions

Unflag import assertions and remove possibility of importing JSON
modules without using an assertion.

Refs: #37375 (comment)

doc/api/errors.md

@@ -1111,6 +1111,14 @@ The JS execution context is not associated with a Node.js environment.
 This may occur when Node.js is used as an embedded library and some hooks
 for the JS engine are not set up properly.
 
+<a id="ERR_FAILED_IMPORT_ASSERTION"></a>
+### `ERR_FAILED_IMPORT_ASSERTION`
+<!-- YAML
+added: REPLACEME
+-->
+
+An import assertion has failed, preventing the specified module to be imported.
+
 <a id="ERR_FALSY_VALUE_REJECTION"></a>
 ### `ERR_FALSY_VALUE_REJECTION`
 
@@ -1662,6 +1670,14 @@ for more information.
 
 An invalid HTTP token was supplied.
 
+<a id="ERR_INVALID_IMPORT_ASSERTION"></a>
+### `ERR_INVALID_IMPORT_ASSERTION`
+<!-- YAML
+added: REPLACEME
+-->
+
+An import assertion is not supported by this version of Node.js.
+
 <a id="ERR_INVALID_IP_ADDRESS"></a>
 ### `ERR_INVALID_IP_ADDRESS`
 
@@ -1913,6 +1929,15 @@ strict compliance with the API specification (which in some cases may accept
 `func(undefined)` and `func()` are treated identically, and the
 [`ERR_INVALID_ARG_TYPE`][] error code may be used instead.
 
+<a id="ERR_MISSING_IMPORT_ASSERTION"></a>
+### `ERR_MISSING_IMPORT_ASSERTION`
+<!-- YAML
+added: REPLACEME
+-->
+
+An attempt was made to import a module without an assertion that requires
+a specific import assertion to be loaded.
+
 <a id="ERR_MISSING_OPTION"></a>
 ### `ERR_MISSING_OPTION`
 

doc/api/esm.md

@@ -7,6 +7,9 @@
 <!-- YAML
 added: v8.5.0
 changes:
+  - version: REPLACEME
+    pr-url: https://github.com/nodejs/node/pull/39921
+    description: Add support for import assertions.
   - version:
     - REPLACEME
     pr-url: https://github.com/nodejs/node/pull/37468
@@ -232,6 +235,25 @@ absolute URL strings.
 import fs from 'node:fs/promises';
 ```
 
+## Import assertions
+<!-- YAML
+added: REPLACEME
+-->
+
+The [Import Assertions proposal][] adds an inline syntax for module import
+statements to pass on more information alongside the module specifier.
+
+```js
+import json from './foo.json' assert { type: "json" };
+await import('foo.json', { assert: { type: "json" } });
+```
+
+Node.js supports the following `type` values:
+
+| `type`   | Resolves to      |
+| -------- | ---------------- |
+| `"json"` | [JSON modules][] |
+
 ## Builtin modules
 
 [Core modules][] provide named exports of their public API. A
@@ -522,9 +544,8 @@ same path.
 
 Assuming an `index.mjs` with
 
-<!-- eslint-skip -->
 ```js
-import packageConfig from './package.json';
+import packageConfig from './package.json' assert { type: 'json' };
 ```
 
 The `--experimental-json-modules` flag is needed for the module
@@ -1355,6 +1376,8 @@ success!
 [Dynamic `import()`]: https://wiki.developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/import#Dynamic_Imports
 [ECMAScript Top-Level `await` proposal]: https://github.com/tc39/proposal-top-level-await/
 [ES Module Integration Proposal for Web Assembly]: https://github.com/webassembly/esm-integration
+[Import Assertions proposal]: https://github.com/tc39/proposal-import-assertions
+[JSON modules]: #json-modules
 [Node.js Module Resolution Algorithm]: #resolver-algorithm-specification
 [Terminology]: #terminology
 [URL]: https://url.spec.whatwg.org/

lib/internal/errors.js

@@ -953,6 +953,9 @@ E('ERR_ENCODING_NOT_SUPPORTED', 'The "%s" encoding is not supported',
   RangeError);
 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_FAILED_IMPORT_ASSERTION', (request, key, expectedValue, actualValue) => {
+  return `Failed to load module "${request}", expected ${key} to be ${JSONStringify(expectedValue)}, got ${JSONStringify(actualValue)} instead`;
+}, TypeError);
 E('ERR_FALSY_VALUE_REJECTION', function(reason) {
   this.reason = reason;
   return 'Promise was rejected with falsy value';
@@ -1250,6 +1253,9 @@ E('ERR_INVALID_FILE_URL_HOST',
 E('ERR_INVALID_FILE_URL_PATH', 'File URL path %s', TypeError);
 E('ERR_INVALID_HANDLE_TYPE', 'This handle type cannot be sent', TypeError);
 E('ERR_INVALID_HTTP_TOKEN', '%s must be a valid HTTP token ["%s"]', TypeError);
+E('ERR_INVALID_IMPORT_ASSERTION',
+  (type, value) => `Invalid ${JSONStringify(type)} import assertion: ${JSONStringify(value)}`,
+  TypeError);
 E('ERR_INVALID_IP_ADDRESS', 'Invalid IP address: %s', TypeError);
 E('ERR_INVALID_MODULE_SPECIFIER', (request, reason, base = undefined) => {
   return `Invalid module "${request}" ${reason}${base ?
@@ -1394,6 +1400,9 @@ E('ERR_MISSING_ARGS',
     }
     return `${msg} must be specified`;
   }, TypeError);
+E('ERR_MISSING_IMPORT_ASSERTION',
+  'Failed to load %s: Node.js requires modules of format "%s" to be loaded ' +
+  'using an assertion "%s" with value "%s"', TypeError);
 E('ERR_MISSING_OPTION', '%s is required', TypeError);
 E('ERR_MODULE_NOT_FOUND', (path, base, type = 'package') => {
   return `Cannot find ${type} '${path}' imported from ${base}`;

lib/internal/modules/cjs/loader.js

@@ -1015,9 +1015,10 @@ function wrapSafe(filename, content, cjsModuleInstance) {
       filename,
       lineOffset: 0,
       displayErrors: true,
-      importModuleDynamically: async (specifier) => {
+      importModuleDynamically: async (specifier, _, importAssertions) => {
         const loader = asyncESM.esmLoader;
-        return loader.import(specifier, normalizeReferrerURL(filename));
+        return loader.import(specifier, normalizeReferrerURL(filename),
+                             importAssertions);
       },
     });
   }
@@ -1030,9 +1031,10 @@ function wrapSafe(filename, content, cjsModuleInstance) {
       '__dirname',
     ], {
       filename,
-      importModuleDynamically(specifier) {
+      importModuleDynamically(specifier, _, importAssertions) {
         const loader = asyncESM.esmLoader;
-        return loader.import(specifier, normalizeReferrerURL(filename));
+        return loader.import(specifier, normalizeReferrerURL(filename),
+                             importAssertions);
       },
     });
   } catch (err) {

lib/internal/modules/esm/loader.js

@@ -6,11 +6,13 @@ require('internal/modules/cjs/loader');
 const {
   Array,
   ArrayIsArray,
+  ArrayPrototypeIncludes,
   ArrayPrototypeJoin,
   ArrayPrototypePush,
   FunctionPrototypeBind,
   FunctionPrototypeCall,
   ObjectCreate,
+  ObjectFreeze,
   ObjectSetPrototypeOf,
   PromiseAll,
   RegExpPrototypeExec,
@@ -20,11 +22,14 @@ const {
 } = primordials;
 
 const {
+  ERR_FAILED_IMPORT_ASSERTION,
   ERR_INVALID_ARG_TYPE,
   ERR_INVALID_ARG_VALUE,
+  ERR_INVALID_IMPORT_ASSERTION,
   ERR_INVALID_MODULE_SPECIFIER,
   ERR_INVALID_RETURN_PROPERTY_VALUE,
   ERR_INVALID_RETURN_VALUE,
+  ERR_MISSING_IMPORT_ASSERTION,
   ERR_UNKNOWN_MODULE_FORMAT
 } = require('internal/errors').codes;
 const { pathToFileURL, isURLInstance } = require('internal/url');
@@ -44,6 +49,10 @@ const { translators } = require(
   'internal/modules/esm/translators');
 const { getOptionValue } = require('internal/options');
 
+const importAssertionTypeCache = new SafeWeakMap();
+const finalFormatCache = new SafeWeakMap();
+const supportedTypes = ObjectFreeze([undefined, 'json']);
+
 /**
  * An ESMLoader instance is used as the main entry point for loading ES modules.
  * Currently, this is a singleton -- there is only one used for loading
@@ -202,33 +211,74 @@ class ESMLoader {
       const { ModuleWrap, callbackMap } = internalBinding('module_wrap');
       const module = new ModuleWrap(url, undefined, source, 0, 0);
       callbackMap.set(module, {
-        importModuleDynamically: (specifier, { url }) => {
-          return this.import(specifier, url);
+        importModuleDynamically: (specifier, { url }, importAssertions) => {
+          return this.import(specifier, url, importAssertions);
         }
       });
 
       return module;
     };
     const job = new ModuleJob(this, url, evalInstance, false, false);
     this.moduleMap.set(url, job);
+    finalFormatCache.set(job, 'module');
     const { module } = await job.run();
 
     return {
       namespace: module.getNamespace(),
     };
   }
 
-  async getModuleJob(specifier, parentURL) {
+  async getModuleJob(specifier, parentURL, importAssertions) {
+    if (!ArrayPrototypeIncludes(supportedTypes, importAssertions.type)) {
+      throw new ERR_INVALID_IMPORT_ASSERTION('type', importAssertions.type);
+    }
+
     const { format, url } = await this.resolve(specifier, parentURL);
     let job = this.moduleMap.get(url);
     // CommonJS will set functions for lazy job evaluation.
     if (typeof job === 'function') this.moduleMap.set(url, job = job());
 
-    if (job !== undefined) return job;
+    if (job != null) {
+      const currentImportAssertionType = importAssertionTypeCache.get(job);
+      if (currentImportAssertionType === importAssertions.type) return job;
+
+      try {
+        // To avoid race conditions, wait for previous module to fulfill first.
+        await job.modulePromise;
+      } catch {
+        // If the other job failed with a different `type` assertion, we got
+        // another chance.
+        job = undefined;
+      }
+
+      if (job !== undefined) {
+        const finalFormat = finalFormatCache.get(job);
+        if (importAssertions.type == null && finalFormat === 'json') {
+          throw new ERR_MISSING_IMPORT_ASSERTION(url, finalFormat,
+                                                 'type', 'json');
+        }
+        if (
+          importAssertions.type == null ||
+              (importAssertions.type === 'json' && finalFormat === 'json')
+        ) return job;
+        throw new ERR_FAILED_IMPORT_ASSERTION(
+          url, 'type', importAssertions.type, finalFormat);
+      }
+    }
 
     const moduleProvider = async (url, isMain) => {
       const { format: finalFormat, source } = await this.load(url, { format });
 
+      if (importAssertions.type === 'json' && finalFormat !== 'json') {
+        throw new ERR_FAILED_IMPORT_ASSERTION(
+          url, 'type', importAssertions.type, finalFormat);
+      }
+      if (importAssertions.type !== 'json' && finalFormat === 'json') {
+        throw new ERR_MISSING_IMPORT_ASSERTION(url, finalFormat,
+                                               'type', 'json');
+      }
+      finalFormatCache.set(job, finalFormat);
+
       const translator = translators.get(finalFormat);
 
       if (!translator) throw new ERR_UNKNOWN_MODULE_FORMAT(finalFormat);
@@ -249,6 +299,7 @@ class ESMLoader {
       inspectBrk
     );
 
+    importAssertionTypeCache.set(job, importAssertions.type);
     this.moduleMap.set(url, job);
 
     return job;
@@ -262,18 +313,19 @@ class ESMLoader {
    * loader module.
    *
    * @param {string | string[]} specifiers Path(s) to the module
-   * @param {string} [parentURL] Path of the parent importing the module
-   * @returns {object | object[]} A list of module export(s)
+   * @param {string} parentURL Path of the parent importing the module
+   * @param {Record<string, Record<string, string>>} importAssertions
+   * @returns {Promise<object | object[]>} A list of module export(s)
    */
-  async import(specifiers, parentURL) {
+  async import(specifiers, parentURL, importAssertions) {
     const wasArr = ArrayIsArray(specifiers);
     if (!wasArr) specifiers = [specifiers];
 
     const count = specifiers.length;
     const jobs = new Array(count);
 
     for (let i = 0; i < count; i++) {
-      jobs[i] = this.getModuleJob(specifiers[i], parentURL)
+      jobs[i] = this.getModuleJob(specifiers[i], parentURL, importAssertions)
         .then((job) => job.run())
         .then(({ module }) => module.getNamespace());
     }

lib/internal/modules/esm/module_job.js

@@ -72,8 +72,8 @@ class ModuleJob {
       // so that circular dependencies can't cause a deadlock by two of
       // these `link` callbacks depending on each other.
       const dependencyJobs = [];
-      const promises = this.module.link(async (specifier) => {
-        const jobPromise = this.loader.getModuleJob(specifier, url);
+      const promises = this.module.link(async (specifier, assertions) => {
+        const jobPromise = this.loader.getModuleJob(specifier, url, assertions);
         ArrayPrototypePush(dependencyJobs, jobPromise);
         const job = await jobPromise;
         return job.modulePromise;

lib/internal/modules/esm/translators.js

@@ -107,8 +107,8 @@ function errPath(url) {
   return url;
 }
 
-async function importModuleDynamically(specifier, { url }) {
-  return asyncESM.esmLoader.import(specifier, url);
+async function importModuleDynamically(specifier, { url }, assertions) {
+  return asyncESM.esmLoader.import(specifier, url, assertions);
 }
 
 function createImportMetaResolve(defaultParentUrl) {

lib/internal/modules/run_main.js

@@ -1,6 +1,7 @@
 'use strict';
 
 const {
+  ObjectCreate,
   StringPrototypeEndsWith,
 } = primordials;
 const CJSLoader = require('internal/modules/cjs/loader');
@@ -46,9 +47,8 @@ function runMainESM(mainPath) {
 
   handleMainPromise(loadESM((esmLoader) => {
     const main = path.isAbsolute(mainPath) ?
-      pathToFileURL(mainPath).href :
-      mainPath;
-    return esmLoader.import(main);
+      pathToFileURL(mainPath).href : mainPath;
+    return esmLoader.import(main, undefined, ObjectCreate(null));
   }));
 }
 

lib/internal/process/esm_loader.js

@@ -1,5 +1,9 @@
 'use strict';
 
+const {
+  ObjectCreate,
+} = primordials;
+
 const {
   ERR_VM_DYNAMIC_IMPORT_CALLBACK_MISSING,
 } = require('internal/errors').codes;
@@ -22,13 +26,14 @@ exports.initializeImportMetaObject = function(wrap, meta) {
   }
 };
 
-exports.importModuleDynamicallyCallback = async function(wrap, specifier) {
+exports.importModuleDynamicallyCallback =
+async function importModuleDynamicallyCallback(wrap, specifier, assertions) {
   const { callbackMap } = internalBinding('module_wrap');
   if (callbackMap.has(wrap)) {
     const { importModuleDynamically } = callbackMap.get(wrap);
     if (importModuleDynamically !== undefined) {
       return importModuleDynamically(
-        specifier, getModuleFromWrap(wrap) || wrap);
+        specifier, getModuleFromWrap(wrap) || wrap, assertions);
     }
   }
   throw new ERR_VM_DYNAMIC_IMPORT_CALLBACK_MISSING();
@@ -69,6 +74,7 @@ async function initializeLoader() {
   const exports = await internalEsmLoader.import(
     customLoaders,
     pathToFileURL(cwd).href,
+    ObjectCreate(null),
   );
 
   // Hooks must then be added to external/public loader

lib/internal/process/execution.js

@@ -82,9 +82,9 @@ function evalScript(name, body, breakFirstLine, print) {
       filename: name,
       displayErrors: true,
       [kVmBreakFirstLineSymbol]: !!breakFirstLine,
-      async importModuleDynamically(specifier) {
-        const loader = await asyncESM.esmLoader;
-        return loader.import(specifier, baseUrl);
+      importModuleDynamically(specifier, _, importAssertions) {
+        const loader = asyncESM.esmLoader;
+        return loader.import(specifier, baseUrl, importAssertions);
       }
     }));
   if (print) {