fix: fall back to require(esm) when CJS parses .js with ESM syntax (#16078)

SimenB · web-flow · commit 44c379285700 · 2026-05-02T00:17:24.000+02:00
diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -18,6 +18,7 @@
 - `[jest-circus, jest-jasmine2]` Include `Error.cause` in JSON `failureMessages` output ([#15949](https://github.com/jestjs/jest/issues/15949))
 - `[jest-runtime]` Improve CJS-from-ESM interop: `__esModule`/Babel default unwrap, broader named-export coverage, and shared CJS singleton across importers ([#16050](https://github.com/jestjs/jest/pull/16050))
 - `[jest-runtime]` Load `.js` files with ESM syntax but no `"type":"module"` marker as native ESM ([#16050](https://github.com/jestjs/jest/pull/16050))
+- `[jest-runtime]` Extend the `.js`-with-ESM-syntax fallback to `require()` on Node v24.9+ - falls back to `require(esm)` when the CJS parser rejects ESM syntax ([#16078](https://github.com/jestjs/jest/pull/16078))
 - `[jest-runtime]` Fix deadlocks and double-evaluation in concurrent ESM and wasm imports ([#16050](https://github.com/jestjs/jest/pull/16050))
 - `[jest-runtime]` Fix error when `require()` is called after the Jest environment has been torn down ([#15951](https://github.com/jestjs/jest/pull/15951))
 
diff --git a/e2e/__tests__/nativeEsmRequireModule.test.ts b/e2e/__tests__/nativeEsmRequireModule.test.ts
@@ -20,6 +20,17 @@ onNodeVersions('>=24.9.0', () => {
     expect(stderr).toContain('4 passed');
     expect(exitCode).toBe(0);
   });
+
+  test('require()/import() fall back to ESM for .js with ESM syntax and no "type":"module" marker', () => {
+    const {exitCode, stderr} = runJest(
+      DIR,
+      ['__tests__/syntax-fallback.test.js'],
+      {nodeOptions: '--experimental-vm-modules --no-warnings'},
+    );
+
+    expect(stderr).toContain('2 passed');
+    expect(exitCode).toBe(0);
+  });
 });
 
 onNodeVersions('<24.9.0', () => {
diff --git a/e2e/native-esm-require-module/__tests__/syntax-fallback.test.js b/e2e/native-esm-require-module/__tests__/syntax-fallback.test.js
@@ -0,0 +1,18 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+'use strict';
+
+test('require() of .js with ESM syntax falls back to require(esm)', () => {
+  const ns = require('../fake-esm-js.js');
+  expect(ns.fakeEsmValue).toBe(123);
+});
+
+test('await import() of .js with ESM syntax falls back to ESM', async () => {
+  const ns = await import('../fake-esm-js.js');
+  expect(ns.fakeEsmValue).toBe(123);
+});
diff --git a/e2e/native-esm-require-module/fake-esm-js.js b/e2e/native-esm-require-module/fake-esm-js.js
@@ -0,0 +1,8 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+export const fakeEsmValue = 123;
diff --git a/packages/jest-runtime/src/__tests__/runtime_esm_cjs_interop.test.ts b/packages/jest-runtime/src/__tests__/runtime_esm_cjs_interop.test.ts
@@ -17,9 +17,15 @@
 import * as path from 'path';
 
 // SyntheticModule is only available when --experimental-vm-modules is active.
-const {SyntheticModule} = require('vm') as typeof import('vm');
+const {SourceTextModule, SyntheticModule} =
+  require('vm') as typeof import('vm');
 const vmAvailable = typeof SyntheticModule === 'function';
 const itVm = vmAvailable ? it : it.skip;
+// `require(esm)` needs the v24.9+ sync ESM core.
+const syncCoreAvailable =
+  // @ts-expect-error - hasAsyncGraph is in Node v24.9+, not yet typed
+  typeof SourceTextModule?.prototype.hasAsyncGraph === 'function';
+const itSyncOnly = syncCoreAvailable ? it : it.skip;
 
 const ROOT_DIR = path.join(__dirname, 'test_esm_interop_root');
 const FROM = path.join(ROOT_DIR, 'test.js');
@@ -125,4 +131,26 @@ describe('Runtime loadCjsAsEsm SyntaxError fallback', () => {
       expect(m.namespace.fakeEsmValue).toBe(123);
     },
   );
+
+  itSyncOnly(
+    'require()s a .js file with ESM syntax that has no "type":"module" marker',
+    async () => {
+      const runtime = await createRuntime(__filename, {rootDir: ROOT_DIR});
+      const ns = runtime.requireModule(FROM, './fake-esm-js.js');
+      expect(ns.fakeEsmValue).toBe(123);
+    },
+  );
+
+  // Runtime SyntaxError from inside the CJS body (vs. a parse-time one)
+  // must not trigger the ESM fallback — surfacing the original error
+  // unchanged is the right behavior.
+  itVm(
+    'does not retry as ESM when the CJS body throws a runtime SyntaxError',
+    async () => {
+      const runtime = await createRuntime(__filename, {rootDir: ROOT_DIR});
+      expect(() =>
+        runtime.requireModule(FROM, './throws-syntaxerror.cjs'),
+      ).toThrow('user-thrown SyntaxError from CJS body');
+    },
+  );
 });
diff --git a/packages/jest-runtime/src/__tests__/test_esm_interop_root/throws-syntaxerror.cjs b/packages/jest-runtime/src/__tests__/test_esm_interop_root/throws-syntaxerror.cjs
@@ -0,0 +1,8 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+throw new SyntaxError('user-thrown SyntaxError from CJS body');
diff --git a/packages/jest-runtime/src/index.ts b/packages/jest-runtime/src/index.ts
@@ -64,6 +64,14 @@ import {
 const esmIsAvailable = typeof SourceTextModule === 'function';
 const supportsDynamicImport = esmIsAvailable;
 
+// Marker used by the CJS-as-ESM SyntaxError fallback paths to distinguish
+// parse-time errors (where retrying as ESM is correct) from runtime errors
+// a user might throw from inside a module body.
+const CJS_PARSE_ERROR = Symbol('jest-runtime CJS parse error');
+const isCjsParseError = (error: unknown): error is Error =>
+  isError(error) &&
+  (error as unknown as Record<symbol, unknown>)[CJS_PARSE_ERROR] === true;
+
 const dataURIRegex =
   /^data:(?<mime>text\/javascript|application\/json|application\/wasm)(?:;(?<encoding>charset=utf-8|base64))?,(?<code>.*)$/;
 
@@ -1663,20 +1671,16 @@ export default class Runtime {
     try {
       synthetic = this._buildCjsAsEsmSyntheticModule(from, modulePath, context);
     } catch (error) {
-      // Use .name check instead of instanceof: the error comes from
-      // vm.compileFunction with a sandbox parsingContext, so its prototype
-      // may cross context boundaries.
-      if ((error as Error).name !== 'SyntaxError') {
+      if (!isCjsParseError(error)) {
         throw error;
       }
       // The file may contain ESM syntax with no ESM marker (.mjs /
       // "type":"module") - try loading as native ESM. If the ESM parser also
       // rejects it, the original CJS error was the genuine one; surface it
       // instead of the (less useful) ESM diagnostic.
-      const cjsSyntaxError = error as Error;
       return this.loadEsmModule(modulePath).catch(esmError => {
         throw isError(esmError) && esmError.name === 'SyntaxError'
-          ? cjsSyntaxError
+          ? error
           : esmError;
       });
     }
@@ -1877,6 +1881,15 @@ export default class Runtime {
       );
     } catch (error) {
       moduleRegistry.delete(modulePath);
+      // Mirror of `loadCjsAsEsm`'s SyntaxError fallback for `require()`.
+      if (supportsSyncEvaluate && isCjsParseError(error)) {
+        try {
+          return this._requireEsmModule<T>(modulePath);
+        } catch (esmError) {
+          if (isError(esmError) && esmError.name === 'SyntaxError') throw error;
+          throw esmError;
+        }
+      }
       throw error;
     }
 
@@ -2629,6 +2642,12 @@ export default class Runtime {
         },
       ) as ModuleWrapper;
     } catch (error: any) {
+      // Tag so callers can distinguish parse-time SyntaxErrors (where the
+      // ESM-syntax-in-CJS fallback applies) from runtime SyntaxErrors a user
+      // might throw from inside a CJS module body.
+      if (isError(error)) {
+        (error as unknown as Record<symbol, unknown>)[CJS_PARSE_ERROR] = true;
+      }
       throw handlePotentialSyntaxError(error);
     }
   }
