initialize state to null if not when not set (#1310)

* initialize state to null if not set by components constructor, fixes #1249

* add changelog

Changelog.md

@@ -2,10 +2,11 @@
 
 ### vNext
 - BREAKING: removed cleanupApolloState as it is no longer needed!
-- Exported getDataFromTree on the client 
+- Exported getDataFromTree on the client
 - Removed `redux` from peer dependencies. [Issue #1223](https://github.com/apollographql/react-apollo/issues/1223) [PR #1224](https://github.com/apollographql/react-apollo/pull/1224)
 - Support arrays being returned from render in SSR [#1158](https://github.com/apollographql/react-apollo/pull/1158)
 - Support passing an updater function to `setState` in SSR mode [#1263](https://github.com/apollographql/react-apollo/pull/1263)
+- Correctly initializes component state as null (not undefined) [#1300](https://github.com/apollographql/react-apollo/pull/1310)
 
 ### 2.0.0-beta.0
 - upgrade to Apollo Client 2.0

src/getDataFromTree.ts

@@ -53,6 +53,8 @@ export function walkTree<Cache>(
       // In case the user doesn't pass these to super in the constructor
       instance.props = instance.props || props;
       instance.context = instance.context || context;
+      // set the instance state to null (not undefined) if not set, to match React behaviour
+      instance.state = instance.state || null;
 
       // Override setState to just change the state, not queue up an update.
       //   (we can't do the default React thing as we aren't mounted "properly"

test/react-web/server/index.test.tsx

@@ -649,6 +649,32 @@ describe('SSR', () => {
         .catch(console.error);
     });
 
+    it('should correctly initialize an empty state to null', () => {
+      class Element extends React.Component<any, any> {
+        render() {
+          expect(this.state).toBeNull();
+        }
+      }
+
+      return getDataFromTree(<Element />);
+    });
+
+    it('should maintain any state set in the element constructor', () => {
+      class Element extends React.Component<any, any> {
+        s;
+        constructor(props) {
+          super(props);
+          this.state = { foo: 'bar' };
+        }
+
+        render() {
+          expect(this.state).toEqual({ foo: 'bar' });
+        }
+      }
+
+      return getDataFromTree(<Element />);
+    });
+
     it('should allow for setting state via an updater function', done => {
       const query = gql`
         query user($id: ID) {