module: handle .mjs in .js handler in CommonJS

This refactors the CommonJS loading a bit to create a center point
that handles source loading (`loadSource`) and make format detection
more consistent to pave the way for future synchronous hooks.

- Handle .mjs in the .js handler, similar to how .cjs has been handled.
- Generate the legacy ERR_REQUIRE_ESM in a getRequireESMError() for
  require(esm) handling (when it's disabled).

PR-URL: nodejs#55590
Refs: nodejs/loaders#198
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Marco Ippolito <marcoippolito54@gmail.com>
Reviewed-By: Juan José Arboleda <soyjuanarbol@gmail.com>

Author: joyeecheung

lib/internal/modules/cjs/loader.js

@@ -100,6 +100,9 @@ const kIsMainSymbol = Symbol('kIsMainSymbol');
 const kIsCachedByESMLoader = Symbol('kIsCachedByESMLoader');
 const kRequiredModuleSymbol = Symbol('kRequiredModuleSymbol');
 const kIsExecuting = Symbol('kIsExecuting');
+
+const kFormat = Symbol('kFormat');
+
 // Set first due to cycle with ESM loader functions.
 module.exports = {
   kModuleSource,
@@ -433,10 +436,6 @@ function initializeCJS() {
   // TODO(joyeecheung): deprecate this in favor of a proper hook?
   Module.runMain =
     require('internal/modules/run_main').executeUserEntryPoint;
-
-  if (getOptionValue('--experimental-require-module')) {
-    Module._extensions['.mjs'] = loadESMFromCJS;
-  }
 }
 
 // Given a module name, and a list of paths to test, returns the first
@@ -646,14 +645,7 @@ function resolveExports(nmPath, request) {
 // We don't cache this in case user extends the extensions.
 function getDefaultExtensions() {
   const extensions = ObjectKeys(Module._extensions);
-  if (!getOptionValue('--experimental-require-module')) {
-    return extensions;
-  }
-  // If the .mjs extension is added by --experimental-require-module,
-  // remove it from the supported default extensions to maintain
-  // compatibility.
-  // TODO(joyeecheung): allow both .mjs and .cjs?
-  return ArrayPrototypeFilter(extensions, (ext) => ext !== '.mjs' || Module._extensions['.mjs'] !== loadESMFromCJS);
+  return extensions;
 }
 
 /**
@@ -1280,10 +1272,6 @@ Module.prototype.load = function(filename) {
   this.paths = Module._nodeModulePaths(path.dirname(filename));
 
   const extension = findLongestRegisteredExtension(filename);
-  // allow .mjs to be overridden
-  if (StringPrototypeEndsWith(filename, '.mjs') && !Module._extensions['.mjs']) {
-    throw new ERR_REQUIRE_ESM(filename, true);
-  }
 
   Module._extensions[extension](this, filename);
   this.loaded = true;
@@ -1327,9 +1315,10 @@ let hasPausedEntry = false;
  * Resolve and evaluate it synchronously as ESM if it's ESM.
  * @param {Module} mod CJS module instance
  * @param {string} filename Absolute path of the file.
+ * @param {string} format Format of the module. If it had types, this would be what it is after type-stripping.
+ * @param {string} source Source the module. If it had types, this would have the type stripped.
  */
-function loadESMFromCJS(mod, filename) {
-  const source = getMaybeCachedSource(mod, filename);
+function loadESMFromCJS(mod, filename, format, source) {
   const cascadedLoader = require('internal/modules/esm/loader').getOrInitializeCascadedLoader();
   const isMain = mod[kIsMainSymbol];
   if (isMain) {
@@ -1483,7 +1472,9 @@ function wrapSafe(filename, content, cjsModuleInstance, format) {
  * `exports`) to the file. Returns exception, if any.
  * @param {string} content The source code of the module
  * @param {string} filename The file path of the module
- * @param {'module'|'commonjs'|undefined} format Intended format of the module.
+ * @param {
+ *    'module'|'commonjs'|'commonjs-typescript'|'module-typescript'
+ * } format Intended format of the module.
  */
 Module.prototype._compile = function(content, filename, format) {
   let moduleURL;
@@ -1505,9 +1496,7 @@ Module.prototype._compile = function(content, filename, format) {
   }
 
   if (format === 'module') {
-    // Pass the source into the .mjs extension handler indirectly through the cache.
-    this[kModuleSource] = content;
-    loadESMFromCJS(this, filename);
+    loadESMFromCJS(this, filename, format, content);
     return;
   }
 
@@ -1560,22 +1549,72 @@ Module.prototype._compile = function(content, filename, format) {
 
 /**
  * Get the source code of a module, using cached ones if it's cached.
+ * After this returns, mod[kFormat], mod[kModuleSource] and mod[kURL] will be set.
  * @param {Module} mod Module instance whose source is potentially already cached.
  * @param {string} filename Absolute path to the file of the module.
- * @returns {string}
+ * @returns {{source: string, format?: string}}
  */
-function getMaybeCachedSource(mod, filename) {
-  // If already analyzed the source, then it will be cached.
-  let content;
-  if (mod[kModuleSource] !== undefined) {
-    content = mod[kModuleSource];
+function loadSource(mod, filename, formatFromNode) {
+  if (formatFromNode !== undefined) {
+    mod[kFormat] = formatFromNode;
+  }
+  const format = mod[kFormat];
+
+  let source = mod[kModuleSource];
+  if (source !== undefined) {
     mod[kModuleSource] = undefined;
   } else {
     // TODO(joyeecheung): we can read a buffer instead to speed up
     // compilation.
-    content = fs.readFileSync(filename, 'utf8');
+    source = fs.readFileSync(filename, 'utf8');
+  }
+  return { source, format };
+}
+
+function reconstructErrorStack(err, parentPath, parentSource) {
+  const errLine = StringPrototypeSplit(
+    StringPrototypeSlice(err.stack, StringPrototypeIndexOf(
+      err.stack, '    at ')), '\n', 1)[0];
+  const { 1: line, 2: col } =
+    RegExpPrototypeExec(/(\d+):(\d+)\)/, errLine) || [];
+  if (line && col) {
+    const srcLine = StringPrototypeSplit(parentSource, '\n')[line - 1];
+    const frame = `${parentPath}:${line}\n${srcLine}\n${StringPrototypeRepeat(' ', col - 1)}^\n`;
+    setArrowMessage(err, frame);
+  }
+}
+
+/**
+ * Generate the legacy ERR_REQUIRE_ESM for the cases where require(esm) is disabled.
+ * @param {Module} mod The module being required.
+ * @param {undefined|object} pkg Data of the nearest package.json of the module.
+ * @param {string} content Source code of the module.
+ * @param {string} filename Filename of the module
+ * @returns {Error}
+ */
+function getRequireESMError(mod, pkg, content, filename) {
+  // This is an error path because `require` of a `.js` file in a `"type": "module"` scope is not allowed.
+  const parent = mod[kModuleParent];
+  const parentPath = parent?.filename;
+  const packageJsonPath = pkg?.path ? path.resolve(pkg.path, 'package.json') : null;
+  const usesEsm = containsModuleSyntax(content, filename);
+  const err = new ERR_REQUIRE_ESM(filename, usesEsm, parentPath,
+                                  packageJsonPath);
+  // Attempt to reconstruct the parent require frame.
+  const parentModule = Module._cache[parentPath];
+  if (parentModule) {
+    let parentSource;
+    try {
+      ({ source: parentSource } = loadSource(parentModule, parentPath));
+    } catch {
+      // Continue regardless of error.
+    }
+    if (parentSource) {
+      // TODO(joyeecheung): trim off internal frames from the stack.
+      reconstructErrorStack(err, parentPath, parentSource);
+    }
   }
-  return content;
+  return err;
 }
 
 /**
@@ -1584,57 +1623,25 @@ function getMaybeCachedSource(mod, filename) {
  * @param {string} filename The file path of the module
  */
 Module._extensions['.js'] = function(module, filename) {
-  // If already analyzed the source, then it will be cached.
-  const content = getMaybeCachedSource(module, filename);
-
-  let format;
-  if (StringPrototypeEndsWith(filename, '.js')) {
-    const pkg = packageJsonReader.readPackageScope(filename) || { __proto__: null };
-    // Function require shouldn't be used in ES modules.
-    if (pkg.data?.type === 'module') {
-      if (getOptionValue('--experimental-require-module')) {
-        module._compile(content, filename, 'module');
-        return;
-      }
-
-      // This is an error path because `require` of a `.js` file in a `"type": "module"` scope is not allowed.
-      const parent = module[kModuleParent];
-      const parentPath = parent?.filename;
-      const packageJsonPath = path.resolve(pkg.path, 'package.json');
-      const usesEsm = containsModuleSyntax(content, filename);
-      const err = new ERR_REQUIRE_ESM(filename, usesEsm, parentPath,
-                                      packageJsonPath);
-      // Attempt to reconstruct the parent require frame.
-      if (Module._cache[parentPath]) {
-        let parentSource;
-        try {
-          parentSource = fs.readFileSync(parentPath, 'utf8');
-        } catch {
-          // Continue regardless of error.
-        }
-        if (parentSource) {
-          const errLine = StringPrototypeSplit(
-            StringPrototypeSlice(err.stack, StringPrototypeIndexOf(
-              err.stack, '    at ')), '\n', 1)[0];
-          const { 1: line, 2: col } =
-              RegExpPrototypeExec(/(\d+):(\d+)\)/, errLine) || [];
-          if (line && col) {
-            const srcLine = StringPrototypeSplit(parentSource, '\n')[line - 1];
-            const frame = `${parentPath}:${line}\n${srcLine}\n${
-              StringPrototypeRepeat(' ', col - 1)}^\n`;
-            setArrowMessage(err, frame);
-          }
-        }
-      }
-      throw err;
-    } else if (pkg.data?.type === 'commonjs') {
-      format = 'commonjs';
-    }
-  } else if (StringPrototypeEndsWith(filename, '.cjs')) {
+  let format, pkg;
+  if (StringPrototypeEndsWith(filename, '.cjs')) {
     format = 'commonjs';
+  } else if (StringPrototypeEndsWith(filename, '.mjs')) {
+    format = 'module';
+  } else if (StringPrototypeEndsWith(filename, '.js')) {
+    pkg = packageJsonReader.readPackageScope(filename) || { __proto__: null };
+    const typeFromPjson = pkg.data?.type;
+    if (typeFromPjson === 'module' || typeFromPjson === 'commonjs' || !typeFromPjson) {
+      format = typeFromPjson;
+    }
   }
-
-  module._compile(content, filename, format);
+  const { source, format: loadedFormat } = loadSource(module, filename, format);
+  // Function require shouldn't be used in ES modules when require(esm) is disabled.
+  if (loadedFormat === 'module' && !getOptionValue('--experimental-require-module')) {
+    const err = getRequireESMError(module, pkg, source, filename);
+    throw err;
+  }
+  module._compile(source, filename, loadedFormat);
 };
 
 /**