Cypress testing pseudo CSS class :before

There's a way to assert on the CSS properties of pseudo-elements, although it's not as simple as just using a Cypress command.

1. Use cy.get() to get a reference to the element.
2. Read the Window object off of the element, and then invoke Window.getComputedStyle(), which can read the computed CSS of pseudo selectors.
3. Use getPropertyValue on the returned CSS declaration to read the value of the content property.
4. Assert on it.

Here's an example that works on the code posted in the OP:

cy.get('.myClass')
.then($els => {
  // get Window reference from element
  const win = $els[0].ownerDocument.defaultView
  // use getComputedStyle to read the pseudo selector
  const before = win.getComputedStyle($els[0], 'before')
  // read the value of the `content` CSS property
  const contentValue = before.getPropertyValue('content')
  // the returned value will have double quotes around it, but this is correct
  expect(contentValue).to.eq('"foo-"')
})

Based on Zach's answer I created a command that returns the pseudo-element property (without single quotes around).

function unquote(str) {
    return str.replace(/(^")|("$)/g, '');
}

Cypress.Commands.add(
    'before',
    {
        prevSubject: 'element',
    },
    (el, property) => {
        const win = el[0].ownerDocument.defaultView;
        const before = win.getComputedStyle(el[0], 'before');
        return unquote(before.getPropertyValue(property));
    },
);

You will use it like this

it('color is black', () => {
    cy.get('button')
       .before('color')
       .should('eq', 'rgb(0,0,0)'); // Or .then()
});