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Differences Between CLI Testing Library & Testing Library

While we clearly take inspiration from DOM Testing Library and React Testing Library, and try to do our best to align as much as possible, there are some major distinctions between the project’s APIs.

Instances

While the DOM Testing Library and it’s descendants have a concept of both a container and element, CLI Testing Library only has a single concept to replace them both: a TestInstance.

This is because, while the DOM makes a clear distinction between different elements (say, HTMLBodyElement and HTMLDivElement), there is no such distinction between processes in the CLI as far as I’m aware.

While this may change in the future (maybe container can be a sort of TestProcess of some kind), there are unanswered questions around what that API looks like and whether we can meaningfully handle the distinction.

Queries

Similarly, HTMLElements provide clean lines between user input items. Meanwhile, the CLI only has the concepts of stdin, stdout, and stderr. It’s unclear how matching a single line of stdout would help in a beneficial matter, much less how APIs like fireEvent would utilize this seemingly extraneous metadata.

As a result, we don’t have any *AllBy queries that DOM Testing Library does. All of our queries are non-plural (*By) and will simply return either the test instance the query was called on, or null depending on the query.

While we would be happy to reconsider, there are once again lots of questions around what *AllBy queries would like and whether we can meaningfully use that concept.

Events

Another area where we diverge from the DOM is in our event system (as implemented via fireEvent and userEvent).

Despite our APIs’ naming indicating an actual event system (complete with bubbling and more, like the DOM), the CLI has no such concept.

FireEvent

This means that, while fireEvent in the DOM Testing Library has the ability to inherent from all baked-into-browser events, we must hard-code a list of “events” and actions a user may take.

We try our best to implement ones that make sense:

  • fireEvent.write({value: str}) to write str into stdin
  • fireEvent.sigkill() to send a sigkill to the process
  • fireEvent.sigint() to send a sigint to the process

There is a missing API for that might make sense in keypress. It’s unclear what this behavior would do that write wouldn’t be able to.

UserEvent

There’s also the API of userEvent that allows us to implement a fairly similar keyboard event to upstream. However, there are a few differences here as well:

  1. userEvent can be bound to the render output. This is due to limitations of not having a screen API. It’s unclear how this would work in practice, due to a lack of the window API.
  2. userEvent.keyboard does not support modifier keys. It’s only key-presses, no holding. This is because of API differences that would require us to figure out a manual binding system for every single key using ANSI AFAIK.
  3. Relatedly, we’ve remove the { syntax support, since there is no standard keybinding for the CLI, like there is for the DOM. Instead, we only support [ and ] syntax.

Matchers

While most of this document has been talking about DOM Testing Library and React Testing Library, let’s take a moment to talk about jest-dom matchers.

Usually, in a Testing Library testbed, you’d expect a getByText to look something like this:

const {getByText} = render(/* Something */)


expect(getByText('Hello World')).toBeInTheDocument()

In today’s version of CLI Testing Library, the same would look something like this:

const {getByText} = render(/* Something */)


expect(getByText('Hello World')).toBeInTheConsole()

Similarities

None of this is to say that we’re not dedicated to being aligned with upstream. We would love to work with the broader Testing Library community to figure out these problems and adjust our usage.

This is the primary reason the library is still published as an @alpha, despite being stable enough to successfully power a popular CLI application’s testbed.

What’s more, we’re dedicated enough to making this happen that we’ve made sure to:

  • Use the same source code organization (as much as possible) as DOM Testing Library and React Testing Library
  • Use the same deployment process as DOM Testing Library
  • Use similar documentation styles

And more!