test-real-styles

(test-)framework agnostic utilities to test real styling of (virtual) dom elements

Motivation

I created this with jest and testing-library in mind to programmatically test appearance effects of component APIs in real browsers.

While @testing-library/jest-dom has a toHaveStyle assertion and there are ways to test css-in-js all solutions I've tried ignore the css cascade or use a buggy/incomplete simulation of it.

But real-world components do use the cascade. This library aims to give you confidence that a style is actually active on an element.

Installation

npm install test-real-styles

Usage

Assuming a NodeJS test environment with jsdom like jest.

import { getRealStyles, launch, toCss, LaunchedPage } from '../index';

const MY_CSS = `
  button { background-color: fuchsia; }
  button::after {
    border: 2px solid olive;
  }
  button:hover { color: #123456; }
  button:focus { color: rgba(255, 0, 123, 0.5); }
`;

describe(`button`, () => {
  it('is pink', async () => {
    /**
     * This will
     * - launch a headless chromium
     * - insert the css
     * - update the page with `doc`
     * - return getComputedStyle(doc)['backgroundColor']
     */
    const styles = await getRealStyles({
      css: MY_CSS,
      doc: document.createElement('button'),
      getStyles: ['backgroundColor'],
    });

    expect(styles).toEqual({ backgroundColor: 'fuchsia' });
  });

  describe('launch API', () => {
    /* In case you want to re-use the browser, interact with the page or do stuff
       before styles are returned */

    let launchedPage: LaunchedPage | null = null;
    beforeAll(() => {
      launchedPage = launch('webkit', MY_CSS);
    });

    it('gets hover and focus styles', async () => {
      const { updatePage, hover, focus, getStyles } = launchedPage!;
      const button = document.createElement('button');
      await updatePage(button);

      await hover(button);
      const hoverStyles = await getStyles(button, ['color', 'backgroundColor']);
      await focus(button);
      const focusStyles = await getStyles(button, ['color']);

      expect(toCss(hoverStyles)).toMatchInlineSnapshot(`
        "color: #123456;
        background-color: fuchsia;"
      `);
      expect(focusStyles.color).toBe('rgba(255, 0, 123, 0.5)');
    });

    it('gets pseudo elements', async () => {
      const { updatePage, getStyles } = launchedPage!;
      const button = document.createElement('button');
      await updatePage(button);

      expect(
        await getStyles(button, ['border'], { pseudoElt: '::after' }),
      ).toEqual({ border: '2px solid olive' });
    });
  });
});

Examples

• Using Sass or other pre-processors
• Using CSS-Modules, React & TestingLibrary
• Using StyledComponents
• Run test in multiple browsers parallel

License

The MIT License

Copyright (C) 2022 Hannes Diercks

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.