esm: add --experimental-import-non-javascript-without-assertion flag

Restricts import of JSON modules to the assertion form only, unless the
`--experimental-import-non-javascript-without-assertion` CLI flag is
provided.

Author: aduh95

doc/api/cli.md

@@ -251,6 +251,14 @@ added:
 
 Enable experimental `import.meta.resolve()` support.
 
+### `--experimental-import-non-javascript-without-assertion`
+<!-- YAML
+added: REPLACEME
+-->
+
+Enable experimental support importing non-JS modules without using an import
+assertion.
+
 ### `--experimental-json-modules`
 <!-- YAML
 added: v12.9.0

doc/api/errors.md

@@ -1925,6 +1925,15 @@ 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 impor without an assertion a module that requires a
+specific import assertion to be loaded.
+
 <a id="ERR_MISSING_OPTION"></a>
 ### `ERR_MISSING_OPTION`
 

lib/internal/errors.js

@@ -1400,6 +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/esm/loader.js

@@ -29,6 +29,7 @@ const {
   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');
@@ -48,6 +49,9 @@ const { translators } = require(
   'internal/modules/esm/translators');
 const { getOptionValue } = require('internal/options');
 
+const experimentalAssertionlessNonJsImports =
+  getOptionValue('--experimental-import-non-javascript-without-assertion');
+
 const importAssertionTypeCache = new SafeWeakMap();
 const finalFormatCache = new SafeWeakMap();
 const supportedTypes = ObjectFreeze([undefined, 'json']);
@@ -252,6 +256,11 @@ class ESMLoader {
 
       if (job !== undefined) {
         const finalFormat = finalFormatCache.get(job);
+        if (!experimentalAssertionlessNonJsImports &&
+           import_assertions.type == null && finalFormat === 'json') {
+          throw new ERR_MISSING_IMPORT_ASSERTION(url, finalFormat,
+                                                 'type', 'json');
+        }
         if (
           import_assertions.type == null ||
               (import_assertions.type === 'json' && finalFormat === 'json')
@@ -268,6 +277,10 @@ class ESMLoader {
         throw new ERR_FAILED_IMPORT_ASSERTION(
           url, 'type', import_assertions.type, finalFormat);
       }
+      if (!experimentalAssertionlessNonJsImports && finalFormat === 'json') {
+        throw new ERR_MISSING_IMPORT_ASSERTION(url, finalFormat,
+                                               'type', 'json');
+      }
       finalFormatCache.set(job, finalFormat);
 
       const translator = translators.get(finalFormat);

src/node_options.cc

@@ -315,6 +315,11 @@ EnvironmentOptionsParser::EnvironmentOptionsParser() {
             "experimental JSON interop support for the ES Module loader",
             &EnvironmentOptions::experimental_json_modules,
             kAllowedInEnvironment);
+  AddOption("--experimental-import-non-javascript-without-assertion",
+            "experimental support for importing non-JS modules without using "
+            "an import assertion",
+            &EnvironmentOptions::experimental_assertionless_non_js_imports,
+            kAllowedInEnvironment);
   AddOption("--experimental-loader",
             "use the specified module as a custom loader",
             &EnvironmentOptions::userland_loader,

src/node_options.h

@@ -104,6 +104,7 @@ class EnvironmentOptions : public Options {
   std::string dns_result_order;
   bool enable_source_maps = false;
   bool experimental_json_modules = false;
+  bool experimental_assertionless_non_js_imports = false;
   bool experimental_modules = false;
   std::string experimental_specifier_resolution;
   bool experimental_wasm_modules = false;

test/es-module/test-esm-data-urls.js

@@ -1,4 +1,4 @@
-// Flags: --experimental-json-modules
+// Flags: --experimental-json-modules --experimental-import-non-javascript-without-assertion
 'use strict';
 const common = require('../common');
 const assert = require('assert');

test/es-module/test-esm-dynamic-import-assertion.js

@@ -1,4 +1,4 @@
-// Flags: --experimental-json-modules
+// Flags: --experimental-json-modules --experimental-import-non-javascript-without-assertion
 'use strict';
 const common = require('../common');
 const { rejects, strictEqual } = require('assert');

test/es-module/test-esm-dynamic-import-assertion.mjs

@@ -1,4 +1,4 @@
-// Flags: --experimental-json-modules
+// Flags: --experimental-json-modules --experimental-import-non-javascript-without-assertion
 import '../common/index.mjs';
 import { rejects, strictEqual } from 'assert';
 

test/es-module/test-esm-import-assertion-3.mjs

@@ -1,4 +1,4 @@
-// Flags: --experimental-json-modules
+// Flags: --experimental-json-modules --experimental-import-non-javascript-without-assertion
 import '../common/index.mjs';
 import { strictEqual } from 'assert';