Fix old test cases and create new for app component

examples/basic-example/app.js

@@ -305,6 +305,7 @@ class App extends Component {
             </button>
             <button 
               onClick={(e) => this.setState({scrollToIndex: scrollToIndex + 1})}
+              id="btnNext"
             >
               next to {scrollToIndex + 1}
             </button>

examples/basic-example/app.test.js

@@ -0,0 +1,39 @@
+import React from 'react';
+import { shallow } from 'enzyme';
+import App from './app';
+
+describe('<App/>',  ()=> {
+
+    let wrapper;
+
+    beforeEach(()=>{
+        wrapper = shallow(<App/>);
+    });
+
+    it('Check length',()=>{
+        expect(wrapper).toHaveLength(1);
+    });
+
+    it('onClick next', ()=>{
+
+        let nextBtn = wrapper.find('#btnNext');
+        expect(nextBtn).toHaveLength(1);
+        expect(wrapper.instance().state.scrollToIndex).toEqual(0);
+        nextBtn.simulate('click');
+        expect(wrapper.instance().state.scrollToIndex).toEqual(1);
+        wrapper.update();
+        nextBtn = wrapper.find('#btnNext');
+        expect(nextBtn.text()).toEqual(`next to ${wrapper.instance().state.scrollToIndex + 1}`);
+        nextBtn.simulate('click');
+        expect(wrapper.instance().state.scrollToIndex).toEqual(2);
+        wrapper.update();
+        nextBtn = wrapper.find('#btnNext');
+        expect(nextBtn.text()).toEqual(`next to ${wrapper.instance().state.scrollToIndex + 1}`);
+
+    });
+
+
+
+
+
+});