Title: Javascript / Interview / SRIQ - Reactjs interview questions - sudheer jonna / Part I - Fundamentals / Chapter 4 - React Testing

Front: What is Jest?
Back: Jest is a JavaScript unit testing framework created by Facebook based on Jasmine and provides automated mock creation and a jsdom environment. It's often used for testing components.

Front: What is the purpose of ReactTestUtils package?
Back: ReactTestUtils are provided in the with-addons package and allow you to perform actions against a simulated DOM for the purpose of unit testing.

Front: What is TestRenderer package in React?
Back: This package provides a renderer that can be used to render components to pure JavaScript objects, without depending on the DOM or a native mobile environment. This package makes it easy to grab a snapshot of the platform view hierarchy (similar to a DOM tree) rendered by a ReactDOM or React Native without using a browser or jsdom.
import TestRenderer from 'react-test-renderer';
const Link = ({ page, children }) => <a href={page}>{children}</a>;
const testRenderer = TestRenderer.create(
    <Link page={'https://www.facebook.com/'}>{'Facebook'}</Link>,
);
console.log(testRenderer.toJSON());
// {
//   type: 'a',
//   props: { href: 'https://www.facebook.com/' },
//   children: [ 'Facebook' ]
// }

Front: What is Shallow Renderer in React testing?
Back: Shallow rendering is useful for writing unit test cases in React. It lets you render a component one level deep and assert facts about what its render method returns, without worrying about the behavior of child components, which are not instantiated or rendered.
For example, if you have the following component:
function MyComponent() {
    return (
        <div>
            <span className={'heading'}>{'Title'}</span>
            <span className={'description'}>{'Description'}</span>
        </div>
    );
}
Then you can assert as follows:
import ShallowRenderer from 'react-test-renderer/shallow';
// in your test
const renderer = new ShallowRenderer();
renderer.render(<MyComponent />);
const result = renderer.getRenderOutput();
expect(result.type).toBe('div');
expect(result.props.children).toEqual([
    <span className={'heading'}>{'Title'}</span>,
    <span className={'description'}>{'Description'}</span>,
]);

Front: Give a simple example of Jest test case
Back: Let's write a test for a function that adds two numbers in sum.js file:
const sum = (a, b) => a + b;
export default sum;
Create a file named sum.test.js which contains actual test:
import sum from './sum';
test('adds 1 + 2 to equal 3', () => {
    expect(sum(1, 2)).toBe(3);
});
And then add the following section to your package.json:
{
    "scripts": {
        "test": "jest"
    }
}
Finally, run yarn test or npm test and Jest will print a result:
$ yarn test
PASS ./sum.test.js
✓ adds 1 + 2 to equal 3 (2ms)

Front: What are the advantages of Jest over Jasmine?
Back: There are couple of advantages compared to Jasmine:
- Automatically finds tests to execute in your source code.
- Automatically mocks dependencies when running your tests.
- Allows you to test asynchronous code synchronously.
- Runs your tests with a fake DOM implementation (via jsdom) so that your tests can be run on the command line.
- Runs tests in parallel processes so that they finish sooner.