Added a warning for adding a property in the SyntheticEvent object

Author: koba04

src/renderers/dom/client/syntheticEvents/SyntheticEvent.js

@@ -17,6 +17,8 @@ var assign = require('Object.assign');
 var emptyFunction = require('emptyFunction');
 var warning = require('warning');
 
+var didWarnAddedProperty = false;
+
 /**
  * @interface Event
  * @see http://www.w3.org/TR/DOM-Level-3-Events/
@@ -157,14 +159,33 @@ assign(SyntheticEvent.prototype, {
    */
   destructor: function() {
     var Interface = this.constructor.Interface;
-    for (var propName in Interface) {
+    var propName;
+    for (propName in Interface) {
       this[propName] = null;
     }
     this.dispatchConfig = null;
     this._targetInst = null;
     this.nativeEvent = null;
-  },
+    this.isDefaultPrevented = null;
+    this.isPropagationStopped = null;
 
+    if (__DEV__) {
+      if (!didWarnAddedProperty) {
+        for (propName in this) {
+          if (this.hasOwnProperty(propName) && this[propName] != null) {
+            warning(
+              didWarnAddedProperty,
+              'This synthetic event is reused for performance reasons. If you\'re ' +
+              'seeing this, you\'re adding a property in the synthetic event object.' +
+              'The property is never released. See ' +
+              'https://fb.me/react-event-pooling for more information.'
+            );
+            didWarnAddedProperty = true;
+          }
+        }
+      }
+    }
+  },
 });
 
 SyntheticEvent.Interface = EventInterface;

src/renderers/dom/client/syntheticEvents/__tests__/SyntheticEvent-test.js

@@ -108,4 +108,19 @@ describe('SyntheticEvent', function() {
       'https://fb.me/react-event-pooling for more information.'
     );
   });
+
+  it('should warn if the synthetic event is added a property', function() {
+    spyOn(console, 'error');
+    var syntheticEvent = createEvent({});
+    syntheticEvent.foo = 'foo';
+    SyntheticEvent.release(syntheticEvent);
+    expect(syntheticEvent.foo).toBe('foo');
+    expect(console.error.calls.length).toBe(1);
+    expect(console.error.argsForCall[0][0]).toBe(
+      'Warning: This synthetic event is reused for performance reasons. If you\'re ' +
+      'seeing this, you\'re adding a property in the synthetic event object.' +
+      'The property is never released. See ' +
+      'https://fb.me/react-event-pooling for more information.'
+    );
+  });
 });