fix(fireEvent): copy non-enumerable data transfer props (#734)

* Copy non enumerable data transfer prop

* Create a test for dataTransfer prop copying

* Make dataTransfer test more generic

Closes #727

Author: Snizhana

src/__tests__/events.js

@@ -295,6 +295,26 @@ test('assigns dataTransfer properties', () => {
   expect(spy.mock.calls[0][0]).toHaveProperty('dataTransfer.dropEffect', 'move')
 })
 
+test('assigns dataTransfer non-enumerable properties', () => {
+  window.DataTransfer = function DataTransfer() {}
+  const node = document.createElement('div')
+  const spy = jest.fn()
+  const item = {};
+  const dataTransfer = new window.DataTransfer();
+
+  Object.defineProperty(dataTransfer, 'items', {
+    value: [item],
+    enumerable: false
+  })
+  node.addEventListener('drop', spy)
+  fireEvent.drop(node, {dataTransfer})
+
+  expect(spy).toHaveBeenCalledTimes(1)
+  expect(spy.mock.calls[0][0].dataTransfer.items).toHaveLength(1)
+
+  delete window.DataTransfer
+})
+
 test('assigning the files property on dataTransfer', () => {
   const node = document.createElement('div')
   const file = new document.defaultView.File(['(⌐□_□)'], 'chucknorris.png', {

src/events.js

@@ -71,7 +71,12 @@ function createEvent(
       /* istanbul ignore if  */
       if (typeof window.DataTransfer === 'function') {
         Object.defineProperty(event, dataTransferKey, {
-          value: Object.assign(new window.DataTransfer(), dataTransferValue),
+          value: Object
+            .getOwnPropertyNames(dataTransferValue)
+            .reduce((acc, propName) => {
+              Object.defineProperty(acc, propName, {value: dataTransferValue[propName]});
+              return acc;
+            }, new window.DataTransfer())
         })
       } else {
         Object.defineProperty(event, dataTransferKey, {