Detect ambiguous files and string input

Author: GeoffreyBooth

lib/internal/modules/esm/get_format.js

@@ -21,6 +21,7 @@ const experimentalNetworkImports =
 const defaultTypeFlag = getOptionValue('--experimental-default-type');
 // The next line is where we flip the default to ES modules someday.
 const defaultType = defaultTypeFlag === 'module' ? 'module' : 'commonjs';
+const { containsModuleSyntax } = internalBinding('contextify');
 const { getPackageType } = require('internal/modules/esm/resolve');
 const { fileURLToPath } = require('internal/url');
 const { ERR_UNKNOWN_FILE_EXTENSION } = require('internal/errors').codes;
@@ -90,20 +91,34 @@ function underNodeModules(url) {
  * @returns {string}
  */
 function getFileProtocolModuleFormat(url, context, ignoreErrors) {
+  const { source } = context;
   const ext = extname(url);
 
   if (ext === '.js') {
     const packageType = getPackageType(url);
     if (packageType !== 'none') {
       return packageType;
     }
+
     // The controlling `package.json` file has no `type` field.
-    if (defaultType === 'module') {
-      // An exception to the type flag making ESM the default everywhere is that package scopes under `node_modules`
-      // should retain the assumption that a lack of a `type` field means CommonJS.
-      return underNodeModules(url) ? 'commonjs' : 'module';
+    switch (getOptionValue('--experimental-default-type')) {
+      case 'module': { // The user explicitly passed `--experimental-default-type=module`.
+        // An exception to the type flag making ESM the default everywhere is that package scopes under `node_modules`
+        // should retain the assumption that a lack of a `type` field means CommonJS.
+        return underNodeModules(url) ? 'commonjs' : 'module';
+      }
+      case 'commonjs': { // The user explicitly passed `--experimental-default-type=commonjs`.
+        return 'commonjs';
+      }
+      default: { // The user did not pass `--experimental-default-type`.
+        // `source` is undefined when this is called from `defaultResolve`;
+        // but this gets called again from `defaultLoad`/`defaultLoadSync`.
+        if (source && getOptionValue('--experimental-detect-module')) {
+          return containsModuleSyntax(`${source}`, fileURLToPath(url)) ? 'module' : 'commonjs';
+        }
+        return 'commonjs';
+      }
     }
-    return 'commonjs';
   }
 
   if (ext === '') {

lib/internal/modules/esm/load.js

@@ -33,7 +33,7 @@ const DATA_URL_PATTERN = /^[^/]+\/[^,;]+(?:[^,]*?)(;base64)?,([\s\S]*)$/;
 /**
  * @param {URL} url URL to the module
  * @param {ESModuleContext} context used to decorate error messages
- * @returns {{ responseURL: string, source: string | BufferView }}
+ * @returns {Promise<{ responseURL: string, source: string | BufferView }>}
  */
 async function getSource(url, context) {
   const { protocol, href } = url;
@@ -127,19 +127,24 @@ async function defaultLoad(url, context = kEmptyObject) {
 
   throwIfUnsupportedURLScheme(urlInstance, experimentalNetworkImports);
 
-  format ??= await defaultGetFormat(urlInstance, context);
-
-  validateAttributes(url, format, importAttributes);
-
-  if (
-    format === 'builtin' ||
-    format === 'commonjs'
-  ) {
+  if (urlInstance.protocol === 'node:') {
     source = null;
   } else if (source == null) {
     ({ responseURL, source } = await getSource(urlInstance, context));
+    context.source = source;
+  }
+
+  if (format == null || format === 'commonjs') {
+    // Now that we have the source for the module, run `defaultGetFormat` again in case we detect ESM syntax.
+    format = await defaultGetFormat(urlInstance, context);
+  }
+
+  if (format === 'commonjs') {
+    source = null; // Let the CommonJS loader handle it (for now)
   }
 
+  validateAttributes(url, format, importAttributes);
+
   return {
     __proto__: null,
     format,
@@ -178,16 +183,17 @@ function defaultLoadSync(url, context = kEmptyObject) {
 
   throwIfUnsupportedURLScheme(urlInstance, false);
 
-  format ??= defaultGetFormat(urlInstance, context);
-
-  validateAttributes(url, format, importAttributes);
-
-  if (format === 'builtin') {
+  if (urlInstance.protocol === 'node:') {
     source = null;
   } else if (source == null) {
     ({ responseURL, source } = getSourceSync(urlInstance, context));
+    context.source = source;
   }
 
+  format ??= defaultGetFormat(urlInstance, context);
+
+  validateAttributes(url, format, importAttributes);
+
   return {
     __proto__: null,
     format,

lib/internal/modules/run_main.js

@@ -4,7 +4,9 @@ const {
   StringPrototypeEndsWith,
 } = primordials;
 
+const { containsModuleSyntax } = internalBinding('contextify');
 const { getOptionValue } = require('internal/options');
+const fs = require('fs');
 const path = require('path');
 
 /**
@@ -70,7 +72,19 @@ function shouldUseESMLoader(mainPath) {
   const { readPackageScope } = require('internal/modules/package_json_reader');
   const pkg = readPackageScope(mainPath);
   // No need to guard `pkg` as it can only be an object or `false`.
-  return pkg.data?.type === 'module' || getOptionValue('--experimental-default-type') === 'module';
+  switch (pkg.data?.type) {
+    case 'module':
+      return true;
+    case 'commonjs':
+      return false;
+    default: { // No package.json or no `type` field.
+      if (getOptionValue('--experimental-detect-module')) {
+        const content = fs.readFileSync(mainPath, 'utf8');
+        return containsModuleSyntax(content, mainPath);
+      }
+      return false;
+    }
+  }
 }
 
 /**

lib/internal/process/execution.js

@@ -26,6 +26,8 @@ const {
   emitAfter,
   popAsyncContext,
 } = require('internal/async_hooks');
+const { containsModuleSyntax } = internalBinding('contextify');
+const { getOptionValue } = require('internal/options');
 const {
   makeContextifyScript, runScriptInThisContext,
 } = require('internal/vm');
@@ -70,6 +72,12 @@ function evalScript(name, body, breakFirstLine, print, shouldLoadESM = false) {
   const baseUrl = pathToFileURL(module.filename).href;
   const { loadESM } = asyncESM;
 
+  if (getOptionValue('--experimental-detect-module') &&
+    getOptionValue('--input-type') === '' && getOptionValue('--experimental-default-type') === '' &&
+    containsModuleSyntax(body, name)) {
+    return evalModule(body, print);
+  }
+
   const runScript = () => {
     // Create wrapper for cache entry
     const script = `