Black Sheep Code

Why I've gone off React Testing Library

React Testing Library is a very popular testing library for React, one that I've known and loved.

React Testing Library's core revolutionary idea, that had it displace Enzyme has the de facto testing tool, boils down to two things:

To be clear - both of these philosophies I agree with.

The problem with React Testing Library is JSDom

JSDOM is a JavaScript implementation of the DOM, in their words:

In general, the goal of the project is to emulate enough of a subset of a web browser to be useful for testing and scraping real-world web applications.

The key issue lies in that word subset.

In my experience there have numerous examples of a piece of code I'm trying to test relying on some browser API that JSDOM has not implemented.

This has us scrambling to write mock implementations like the one suggested in this Stack Overflow answer.

Examples of unsupported browser APIs

HTML5 Form Validation

For example if we have a component that returns:

    <input type="text" required"/>

When we submit this form, in a real browser you will see a browser native validation error and no form submission event will be fired.

JSDOM does not implement this API, and the form submission event will be fired. We won't be able to this 'the form doesn't submit' path.

Resize Observer

For example I was using the React Flow library, which happens to use ResizeObserver. Any component rendered using RTL simply errors.


contenteditable is an API that allows an arbitrary element, like a div, to behave as a textarea. This is useful for creating rich text editors.

JSDOM does not support the contenteditable API.

The rich text editor library Easy Markdown Editor has on option to use contenteditable, but if we do we won't be able to use RTL to test it.

I write more about testing EasyMDE here.

What I suggest using instead - Cypress (or Playwright) component tests

Cypress now supports component tests. Paired with Cypress Testing Library this gives us the original philosophy of RTL: Don't test implementation details and write test interactions as the user would.

Playwright also supports component tests.

Assertions on functions being called, à la jest.fn() can be achieve with Cypress's spy.

Cypress also has the benefit of having a visual display of the test running - something that can be very handy debugging your tests.

An alternative solution - React Testing Library + Karma?

I'm been made aware of a tool called Karma which appears to be a general purpose test runner that runs in the browser.

There is discussion of using this with React Testing Library here:

However, it doesn't otherwise seem particularly well documented.

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