esm: add experimental .json support to loader

With the new flag `--experimental-json-modules` it is now possible
to import .json files. It piggy backs on the current cjs loader
implementation, so it only exports a default. This is a bit of a
hack, and it should potentially have it's own loader, especially
if we change the cjs loader at all.

The behavior for .json in the cjs loader matches the current
planned behavior if json modules were to be standardized, specifically
that a .json module only exports a default.

Refs: nodejs/modules#255
Refs: whatwg/html#4315
Refs: WICG/webcomponents#770

Author: MylesBorins

lib/internal/modules/esm/default_resolve.js

@@ -7,6 +7,7 @@ const { realpathSync } = require('fs');
 const { getOptionValue } = require('internal/options');
 const preserveSymlinks = getOptionValue('--preserve-symlinks');
 const preserveSymlinksMain = getOptionValue('--preserve-symlinks-main');
+const experimentalJsonModules = getOptionValue('--experimental-json-modules');
 const { ERR_UNKNOWN_FILE_EXTENSION } = require('internal/errors').codes;
 const { resolve: moduleWrapResolve } = internalBinding('module_wrap');
 const { pathToFileURL, fileURLToPath } = require('internal/url');
@@ -25,11 +26,20 @@ const legacyExtensionFormatMap = {
   '__proto__': null,
   '.cjs': 'cjs',
   '.js': 'cjs',
-  '.json': 'cjs',
   '.mjs': 'esm',
   '.node': 'cjs'
 };
 
+if (experimentalJsonModules) {
+  // This is a total hack
+  Object.assign(extensionFormatMap, {
+    '.json': 'cjs'
+  });
+  Object.assign(legacyExtensionFormatMap, {
+    '.json': 'cjs'
+  });
+}
+
 function resolve(specifier, parentURL) {
   if (NativeModule.canBeRequiredByUsers(specifier)) {
     return {

src/node_options.cc

@@ -160,6 +160,10 @@ DebugOptionsParser::DebugOptionsParser() {
 }
 
 EnvironmentOptionsParser::EnvironmentOptionsParser() {
+  AddOption("--experimental-json-modules",
+            "experimental JSON interop support for the ES Module loader",
+            &EnvironmentOptions::experimental_json_modules,
+            kAllowedInEnvironment);
   AddOption("--experimental-modules",
             "experimental ES Module support and caching modules",
             &EnvironmentOptions::experimental_modules,

src/node_options.h

@@ -85,6 +85,7 @@ class DebugOptions : public Options {
 class EnvironmentOptions : public Options {
  public:
   bool abort_on_uncaught_exception = false;
+  bool experimental_json_modules = false;
   bool experimental_modules = false;
   std::string module_type;
   std::string experimental_policy;

test/es-module/test-esm-json.mjs

@@ -0,0 +1,9 @@
+// Flags: --experimental-modules --experimental-json-modules
+/* eslint-disable node-core/required-modules */
+
+import '../common/index.mjs';
+import { strictEqual } from 'assert';
+
+import secret from '../fixtures/utf8-bom.json';
+
+strictEqual(secret, 42);