NewMessageForm.spec.js.diff

   describe('clicking send', () => {
     const messageText = 'Hello world';

+    let sendHandler;
     let getByTestId;

     beforeEach(() => {
+      sendHandler = jest.fn();

-      ({ getByTestId } = render(<NewMessageForm />));
+      ({ getByTestId } = render(<NewMessageForm onSend={sendHandler} />));

       fireEvent.changeText(getByTestId('messageText'), messageText);
       fireEvent.press(getByTestId('sendButton'));
...
     it('clears the message field', () => {
       expect(getByTestId('messageText').props.value).toEqual('');
     });
+
+    it('calls the send handler', () => {
+      expect(sendHandler).toHaveBeenCalledWith(messageText);
+    });
   });
 });