I love using Jest Snapshot testing for it’s simplicity when it comes to updating the unit tests. However, with great power come great responsibilities. One essential aspect of it is that you need to review the snapshots generated really thorough. Otherwise, your unit tests could be recording bugs as snapshots.

So, really, snapshot testing only makes it easier to update the expected outcome of the tests, it doesn’t validate the their correctness.

When it comes to snapshot test React components a key aspect is to render the component into a serializable form. And it turns out you can do that using mainly 2 techniques:

Snapshot testing with react-test-renderer

This is a special React components renderer, created by the React team, that allows one to get a JSON representation of a React component. This JSON representation can later be saved as a component snapshot.

import * as React from 'react';
import * as renderer from 'react-test-renderer';
import { CountDown } from './CountDown';

test('should render correctly', () => {
        const tree = renderer.create(
            <CountDown
                startTime={30}
            />
        ).toJSON();
        expect(tree).toMatchSnapshot();
 });

All it’s fine using it, if all you need to unit test are simple rendering tests or very simple interaction tests. The provided react-test-renderer doesn’t have an elegant, built in way of rendering and interacting with a component(clicking items, changing text);

As such I prefer in using Enzyme for all the React unit testing needs.

I’ve been using it before and after snapshot testing came in and with and without Jest framework and it was awesome

Using snapshot testing with Enzyme and Enzyme-to-JSON

The same test reads pretty much the same, but now the rendering is done by Enzyme.

This means you have the full power of the Enzyme API, including the ability to query elements, get props, set state and interact with various elements in the page.

import * as React from 'react';
import { shallow } from 'enzyme';
import toJson from 'enzyme-to-json';
import { CountDown } from './CountDown';

test('should render correctly', () => {
        const wrapper = shallow(
            <CountDown
                startTime={30}
            />
        ).toJSON();
        expect(toJson(wrapper)).toMatchSnapshot();
 });

At this point, you may ask, why enzyme-to-json ?

Fair question. You can go testing even without it, but it turns that the snapshot test will serialize the whole Enzyme wrapper, which is a ton of gibberish that is not relevant to your test.

Using enzyme-to-json ensures that you get more focused tests: only the component own markup gets serialized into the snapshot. That also means when the test fails you get a much more focused context to what exactly was the failure.

An added benefit is that besides shallow rendering you can use mount and then serialize the component with enzyme-to-json.

Hope you enjoyed and I’ve managed to convince you that is not a matter of Jest snapshots vs Enzyme, but a matter of Jest snapshots & Enzyme .

How do you go about unit testing your React components ? Let me know in the comments.

Till next time, stay tuned!