feat(events): Add dataTransfer event property option

Author: jamsinclair

src/__tests__/events.js

@@ -282,6 +282,27 @@ test('assigning the files property on an input', () => {
   expect(node.files).toEqual([file])
 })
 
+test('assigns dataTransfer properties', () => {
+  const node = document.createElement('div')
+  const spy = jest.fn()
+  node.addEventListener('dragover', spy)
+  fireEvent.dragOver(node, {dataTransfer: {dropEffect: 'move'}})
+  expect(spy).toHaveBeenCalledTimes(1)
+  expect(spy.mock.calls[0][0]).toHaveProperty('dataTransfer.dropEffect', 'move')
+})
+
+test('assigning the files property on dataTransfer', () => {
+  const node = document.createElement('div')
+  const file = new document.defaultView.File(['(⌐□_□)'], 'chucknorris.png', {
+    type: 'image/png',
+  })
+  const spy = jest.fn()
+  node.addEventListener('drop', spy)
+  fireEvent.drop(node, {dataTransfer: {files: [file]}})
+  expect(spy).toHaveBeenCalledTimes(1)
+  expect(spy.mock.calls[0][0]).toHaveProperty('dataTransfer.files', [file])
+})
+
 test('fires events on Window', () => {
   const messageSpy = jest.fn()
   window.addEventListener('message', messageSpy)

src/events.js

@@ -44,19 +44,28 @@ Object.keys(eventMap).forEach(key => {
     Object.assign(node, targetProperties)
     const window = getWindowFromNode(node)
     const EventConstructor = window[EventType] || window.Event
+    let event
     /* istanbul ignore else  */
     if (typeof EventConstructor === 'function') {
-      return new EventConstructor(eventName, eventInit)
+      event = new EventConstructor(eventName, eventInit)
     } else {
       // IE11 polyfill from https://developer.mozilla.org/en-US/docs/Web/API/CustomEvent/CustomEvent#Polyfill
-      const event = window.document.createEvent(EventType)
+      event = window.document.createEvent(EventType)
       const {bubbles, cancelable, detail, ...otherInit} = eventInit
       event.initEvent(eventName, bubbles, cancelable, detail)
       Object.keys(otherInit).forEach(eventKey => {
         event[eventKey] = otherInit[eventKey]
       })
-      return event
     }
+
+    // Approximate dataTransfer on the event object
+    // jsdom does not support DataTransfer constructor
+    // https://github.com/testing-library/react-testing-library/issues/339#issuecomment-526310225
+    const {dataTransfer} = eventInit
+    if (typeof dataTransfer === 'object') {
+      Object.assign(event, {dataTransfer})
+    }
+    return event
   }
 
   fireEvent[key] = (node, init) => fireEvent(node, createEvent[key](node, init))