button.test.js

import React from 'react';
import { shallow } from 'enzyme';
import ParentComponent from './index';

it('Test for the button element onChange function', () => {
  //To render the root node and returns a shallow wrapper for ParentComponent
  const wrapper = shallow(<ParentComponent/>);
  //To find the button element 
  const buttonWrapper = wrapper.find('button');
  //To call the onChange function on button named propFunction, with preventDefault
  buttonWrapper.simulate('onChange', { preventDefault: jest.fn() });
});