Detect ESM syntax in --eval

Author: GeoffreyBooth

lib/internal/modules/helpers.js

@@ -5,13 +5,15 @@ const {
   ArrayPrototypeJoin,
   ArrayPrototypeSome,
   ObjectDefineProperty,
+  ObjectGetPrototypeOf,
   ObjectPrototypeHasOwnProperty,
   SafeMap,
   SafeSet,
   StringPrototypeCharCodeAt,
   StringPrototypeIncludes,
   StringPrototypeSlice,
   StringPrototypeStartsWith,
+  SyntaxErrorPrototype,
 } = primordials;
 const {
   ERR_INVALID_ARG_TYPE,
@@ -320,10 +322,29 @@ function hasEsmSyntax(code) {
     stmt.type === 'ExportAllDeclaration');
 }
 
+/**
+ * Check if the error is a syntax error due to ESM syntax in CommonJS.
+ * - `import` statements return an error with a message `Cannot use import statement outside a module`.
+ * - `export` statements return an error with a message `Unexpected token 'export'`.
+ * - `import.meta` returns an error with a message `Cannot use 'import.meta' outside a module`.
+ * Top-level `await` currently returns the same error message as when `await` is used in a sync function,
+ * so we don't use it as a disambiguation.
+ * Dynamic `import()` is permitted in CommonJS, so we don't use it as a disambiguation.
+ * @param {Error} err
+ */
+function isModuleSyntaxError(err) {
+  return err != null && ObjectGetPrototypeOf(err) === SyntaxErrorPrototype && (
+    err.message === 'Cannot use import statement outside a module' ||
+    err.message === "Unexpected token 'export'" ||
+    err.message === "Cannot use 'import.meta' outside a module"
+    );
+}
+
 module.exports = {
   addBuiltinLibsToObject,
   getCjsConditions,
   initializeCjsConditions,
+  isModuleSyntaxError,
   hasEsmSyntax,
   loadBuiltinModule,
   makeRequireFunction,

lib/internal/process/execution.js

@@ -79,19 +79,30 @@ function evalScript(name, body, breakFirstLine, print, shouldLoadESM = false) {
     `;
     globalThis.__filename = name;
     RegExpPrototypeExec(/^/, ''); // Necessary to reset RegExp statics before user code runs.
-    const result = module._compile(script, `${name}-wrapper`)(() =>
-      require('vm').runInThisContext(body, {
-        filename: name,
-        displayErrors: true,
-        [kVmBreakFirstLineSymbol]: !!breakFirstLine,
-        importModuleDynamically(specifier, _, importAssertions) {
-          const loader = asyncESM.esmLoader;
-          return loader.import(specifier, baseUrl, importAssertions);
-        },
-      }));
-    if (print) {
-      const { log } = require('internal/console/global');
-      log(result);
+    try {
+      const result = module._compile(script, `${name}-wrapper`)(() =>
+        require('vm').runInThisContext(body, {
+          filename: name,
+          displayErrors: true,
+          [kVmBreakFirstLineSymbol]: !!breakFirstLine,
+          importModuleDynamically(specifier, _, importAssertions) {
+            const loader = asyncESM.esmLoader;
+            return loader.import(specifier, baseUrl, importAssertions);
+          },
+        }));
+      if (print) {
+        const { log } = require('internal/console/global');
+        log(result);
+      }
+    } catch (error) {
+      const { getOptionValue } = require('internal/options');
+      if (getOptionValue('--experimental-detect-module') && getOptionValue('--input-type') === '' && getOptionValue('--experimental-default-type') === '') {
+        const { isModuleSyntaxError } = require('internal/modules/helpers');
+        if (isModuleSyntaxError(error)) {
+          return evalModule(body, print);
+        }
+      }
+      throw error;
     }
 
     if (origModule !== undefined)

test/es-module/test-esm-detect-ambiguous.mjs

@@ -0,0 +1,18 @@
+import { spawnPromisified } from '../common/index.mjs';
+import { describe, it } from 'node:test';
+import { strictEqual } from 'node:assert';
+
+describe('--experimental-detect-module', () => {
+  it('permits ESM syntax in --eval input without requiring --input-type=module', async () => {
+    const { stdout, stderr, code, signal } = await spawnPromisified(process.execPath, [
+      '--experimental-detect-module',
+      '--eval',
+      'import { version } from "node:process"; console.log(version);',
+    ]);
+
+    strictEqual(stderr, '');
+    strictEqual(stdout, `${process.version}\n`);
+    strictEqual(code, 0);
+    strictEqual(signal, null);
+  });
+})