Cypress assertion to compare colors

support.js

const compareColor = (color, property) => (targetElement) => {
    const tempElement = document.createElement('div');
    tempElement.style.color = color;
    tempElement.style.display = 'none'; // make sure it doesn't actually render
    document.body.appendChild(tempElement); // append so that `getComputedStyle` actually works
  
    const tempColor = getComputedStyle(tempElement).color;
    const targetColor = getComputedStyle(targetElement[0])[property];
  
    document.body.removeChild(tempElement); // remove it because we're done with it
  
    expect(tempColor).to.equal(targetColor);
};

Cypress.Commands.overwrite('should', (originalFn, subject, expectation, ...args) => {
    const customMatchers = {
        'have.backgroundColor': compareColor(args[0], 'backgroundColor'),
        'have.color': compareColor(args[0], 'color'),
    };
  
    // See if the expectation is a string and if it is a member of Jest's expect
    if (typeof expectation === 'string' && customMatchers[expectation]) {
        return originalFn(subject, customMatchers[expectation]);
    }
    return originalFn(subject, expectation, ...args);
});

support.ts

const compareColor = (color: string, property: keyof CSSStyleDeclaration) => (
  targetElement: JQuery
) => {
  const tempElement = document.createElement('div');
  tempElement.style.color = color;
  tempElement.style.display = 'none'; // make sure it doesn't actually render
  document.body.appendChild(tempElement); // append so that `getComputedStyle` actually works

  const tempColor = getComputedStyle(tempElement).color;
  const targetColor = getComputedStyle(targetElement[0])[property];

  document.body.removeChild(tempElement); // remove it because we're done with it

  expect(tempColor).to.equal(targetColor);
};

Cypress.Commands.overwrite('should', (originalFn: Function, subject: any, expectation: any, ...args: any[]) => {
  const customMatchers = {
    'have.backgroundColor': compareColor(args[0], 'backgroundColor'),
    'have.color': compareColor(args[0], 'color'),
  };
  // See if the expectation is a string and if it is a member of Jest's expect
  if (typeof expectation === 'string' && customMatchers[expectation]) {
    return originalFn(subject, customMatchers[expectation]);
  }

  return originalFn(subject, expectation, ...args);
});