Testing Opinions

It happens often at work that my colleagues and I discuss the best way or the proper way to test a specific React component. We don't always agree.

Testing the output of a function is not the same as testing its implementation details.

Avoid Testing Implementation Details

(Almost) all of us agree that testing implementation details should be avoided. The problem sometimes is to agree to a definition of implementation details.

Imagine we have a <Banana> component (of course).

export function Banana({ type, color, length }) {
  return (
    <div className="banana">
      <BananaType type={type} />
      <BananaColor color={color} />
      <BananaLength length={length} />
    </div>
  );
}

Unit test purists would argue we need to test each component in isolation. For the <Banana> component isolation we need to either mock the children or do a shallow render and see how it sets the props in its children. More or less like this:

  it("renders a banana properly", () => {
    const banana = { type: "Cavendish", color: "yellow", length: 12 };
    const wrapper = shallow(<Banana {...banana} />); // from 'enzyme'

    const bananaType = wrapper.find(BananaType); // from 'enzyme'
    const bananaColor = wrapper.find(BananaColor);
    const bananaLength = wrapper.find(BananaLength);

    // make sure each child has the correct props
    expect(bananaType.props()).toHaveProperty("type", "Cavendish");
    expect(bananaColor.props()).toHaveProperty("color", "yellow");
    expect(bananaLength.props()).toHaveProperty("length", 12);
    // 🤔
  });

What's the problem here?

1. The test tries to find BananaType, BananaColor and BananaLength which are implementation dependencies of the subject.
2. The test expects certain props to be set on the internal children; again: implementation details.

What can we do instead?

Our <Banana> component owns those three sub components, so the "subject under test" can be considered to be the entire thing: the component and its children.

I prefer to "render" the component and inspect its output.

  it("renders a banana properly", () => {
    const banana = { type: "Cavendish", color: "yellow", length: 12 };

    const {
      getByText
    } = render(<Banana {...banana} />); // from 'react-testing-library'

    getByText('Cavendish Banana');
    getByText('yellow');
    getByText('12cm');
  });

In some cases a snapshot test may be better:

  it("renders a banana properly", () => {
    const banana = { type: "Cavendish", color: "yellow", length: 12 };
    const result = render(<Banana {...banana} />); // from 'react-testing-library'
    expect(result.container).toMatchSnapshot();
  });

Wait! Don't snapshot tests depend on implementation details?

Some argue that testing against the rendered output (i.e. snapshot test) is similar to testing implementation details. For instance suppose our <Banana> component changes its implementation and instead of rendering <BananaLength> it renders <div class="banana-length">12 cm</div> (notice the space between '12' and 'cm'). Then my tests above would fail.

But I consider the rendered markup to be the function's output, not it's implementation details. If the render changes, then the output of my module changes, then the tests must break.

About react-testing-library

I'm using reac-testing-library in some projects. But the first example was using Enzyme instead. Of course, you can avoid testing implementation details with Enzyme, but I think react-testing-library makes it easier to avoid those bad practices.

My opinions are probably biased because I've been following Kent C. Dodds' testing javascript course.

Feel free to disagree with me in the comments.