You can easily do that with react-testing-library.

First, you have to understand that container or the result of getByText etc. are merely DOM nodes. You can interact with them in the same way you would do in a browser.

So, if you want to know what class is applied to container.firstChild you can just do it like this container.firstChild.className.

If you read more about className in MDN you'll see that it returns all the classes applied to your element separated by a space, that is:

<div class="foo">     => className === 'foo'
<div class="foo bar"> => className === 'foo bar'

This might not be the best solution depending on your case. No worries, you can use another browser API, for example classList.

expect(container.firstChild.classList.contains('foo')).toBe(true)

That's it! No need to learn a new API that works only for tests. It's just as in the browser.

If checking for a class is something you do often you can make the tests easier by adding jest-dom to your project.

The test then becomes:

expect(container.firstChild).toHaveClass('foo')

There are a bunch of other handy methods like toHaveStyle that could help you.

As a side note, react-testing-library is a proper JavaScript testing utility. It has many advantages over other libraries. I encourage you to join the spectrum forum if you're new to JavaScript testing.

The library gives access to normal DOM selectors, so we can also simply do this:

it('Renders with a className equal to the variant', () => {
    const { container } = render(<Button variant="default" />)
    expect(container.getElementsByClassName('default').length).toBe(1);
});

You need to understand the philosophy behind react-testing-library to understand what you can do and what you can't do with it;

The goal behind react-testing-library is for the tests to avoid including implementation details of your components and rather focus on writing tests that give you the confidence for which they are intended.

So querying element by classname is not aligned with the react-testing-library philosophy as it includes implementation details. The classname is actual the implementation detail of an element and is not something the end user will see, and it is subjected to change at anytime in the lifecycle of the element.

So instead of searching element by what user cannot see, and something that can change at anytime, just try to search by using something that the user can see, such as text, label or something that will remain constant in the life cycle of the element like data-id.

So to answer your question, it is not advised to test classname and hence you cannot do that with react-testing-library. Try with other test libraries such as Enzyme or react-dom test utils.