feat(fireEvent): helper to assign target properties to node

Specifically this is so the change event can set the value of the node
in a way that works for React (and all other frameworks and
non-frameworks) nicely.

fixes: testing-library/react-testing-library#152

src/__tests__/events.js

@@ -149,3 +149,13 @@ describe(`Aliased Events`, () => {
     expect(spy).toHaveBeenCalledTimes(1)
   })
 })
+
+test('assigns target properties', () => {
+  const node = document.createElement('input')
+  const spy = jest.fn()
+  const value = 'a'
+  node.addEventListener('change', spy)
+  fireEvent.change(node, {target: {value}})
+  expect(spy).toHaveBeenCalledTimes(1)
+  expect(node.value).toBe(value)
+})

src/events.js

@@ -322,13 +322,37 @@ Object.entries(eventMap).forEach(([key, {EventType = Event, defaultInit}]) => {
 
   fireEvent[key] = (node, init) => {
     const eventInit = Object.assign({}, defaultInit, init)
+    const {target: {value, ...targetProperties} = {}} = eventInit
+    Object.assign(node, targetProperties)
+    if (value !== undefined) {
+      setNativeValue(node, value)
+    }
     const event = new EventType(eventName, eventInit)
     return fireEvent(node, event)
   }
 })
 
+// function written after some investigation here:
+// https://github.com/facebook/react/issues/10135#issuecomment-401496776
+function setNativeValue(element, value) {
+  const {set: valueSetter} =
+    Object.getOwnPropertyDescriptor(element, 'value') || {}
+  const prototype = Object.getPrototypeOf(element)
+  const {set: prototypeValueSetter} =
+    Object.getOwnPropertyDescriptor(prototype, 'value') || {}
+  if (prototypeValueSetter && valueSetter !== prototypeValueSetter) {
+    prototypeValueSetter.call(element, value)
+  } else if (valueSetter) {
+    valueSetter.call(element, value)
+  } else {
+    throw new Error('The given element does not have a value setter')
+  }
+}
+
 Object.entries(eventAliasMap).forEach(([aliasKey, key]) => {
   fireEvent[aliasKey] = (...args) => fireEvent[key](...args)
 })
 
 export {fireEvent}
+
+/* eslint complexity:["error", 9] */