Support SSR

src/Transition.js

@@ -394,7 +394,7 @@ Transition.propTypes = {
  * (see
  * [test/CSSTransition-test.js](https://github.com/reactjs/react-transition-group/blob/13435f897b3ab71f6e19d724f145596f5910581c/test/CSSTransition-test.js#L362-L437)).
  */
- nodeRef: PropTypes.shape({ current: PropTypes.instanceOf(Element) }),
+ nodeRef: PropTypes.shape({ current: PropTypes.instanceOf(typeof Element === 'undefined' ? function() {} : Element) }),
 
  /**
  * A `function` child can be used instead of a React element. This function is

test/SSR-test.js

@@ -0,0 +1,10 @@
+/**
+ * @jest-environment node
+ */
+
+// test that import does not crash
+import * as ReactTransitionGroup from '../src'; // eslint-disable-line no-unused-vars
+
+describe('SSR', () => {
+ it('should import react-transition-group in node env', () => {});
+});