App.test.js

import React from 'react';
import { shallow } from 'enzyme';

// Subject Under Test
import App from './App';

const app = shallow(<App />);

test('Renders nested components', () => {
    expect(app.find('Header').length).toEqual(1);
    expect(app.find('Intro').length).toEqual(1);
    expect(app.find('Students').length).toEqual(1);
    expect(app.find('Summary').length).toEqual(1);
});

test('Can add a student', () => {
    const add = app.find('Students').first();
    const mockNewStudent = {
        name: 'Jimmy Johns',
        grade: 65
    };
    add.props().addStudent(mockNewStudent);

    const addStudent = jest.fn();


    expect(addStudent).toBeCalled();
});