Don't report errors errors as unhandled if there is an existing "error" event handler

integration_tests/__tests__/nested_event_loop.test.js

@@ -0,0 +1,16 @@
+/**
+ * Copyright (c) 2014-present, Facebook, Inc. All rights reserved.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow
+ */
+'use strict';
+
+const runJest = require('../runJest');
+
+test('works with nested event loops', () => {
+  const result = runJest('nested_event_loop');
+  expect(result.status).toBe(0);
+});

integration_tests/nested_event_loop/__tests__/nested_event_loop.test.js

@@ -0,0 +1,31 @@
+/**
+ * Copyright (c) 2014-present, Facebook, Inc. All rights reserved.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @jest-environment jsdom
+ */
+
+/* eslint-env browser */
+
+'use strict';
+
+it('can assert on errors across nested event loops', () => {
+  const el = document.createElement('div');
+  el.addEventListener('fake', () => {
+    throw new Error('This should be caught.');
+  });
+  let caught = null;
+  window.addEventListener('error', e => {
+    caught = e.error;
+  });
+  expect(() => {
+    const evt = document.createEvent('Event');
+    evt.initEvent('fake', false, false);
+    el.dispatchEvent(evt);
+    if (caught) {
+      throw caught;
+    }
+  }).toThrow();
+});

integration_tests/nested_event_loop/package.json

@@ -0,0 +1,5 @@
+{
+  "jest": {
+    "testEnvironment": "jsdom"
+  }
+}

packages/jest-environment-jsdom/src/index.js

@@ -44,13 +44,32 @@ class JSDOMEnvironment {
       };
     }
 
+    // Report uncaught errors.
     this.errorEventListener = event => {
-      if (event.error) {
+      if (userErrorListenerCount === 0 && event.error) {
         process.emit('uncaughtException', event.error);
       }
     };
     global.addEventListener('error', this.errorEventListener);
 
+    // However, don't report them as uncaught if the user listens to 'error' event.
+    // In that case, we assume the might have custom error handling logic.
+    const originalAddListener = global.addEventListener;
+    const originalRemoveListener = global.removeEventListener;
+    let userErrorListenerCount = 0;
+    global.addEventListener = function(name) {
+      if (name === 'error') {
+        userErrorListenerCount++;
+      }
+      return originalAddListener.apply(this, arguments);
+    };
+    global.removeEventListener = function(name) {
+      if (name === 'error') {
+        userErrorListenerCount--;
+      }
+      return originalRemoveListener.apply(this, arguments);
+    };
+
     this.moduleMocker = new mock.ModuleMocker(global);
 
     const timerConfig = {