StylePropertyMapReadOnly.get()

var declarationBlock = StylePropertyMapReadOnly.get(property)

<p>
   <a href="https://example.com">Link</a>
</p>
<dl id="results"></dl>

p {
  font-weight: bold;
}
a {
   --color: red;
   color: var(--color);
}

// get the element
const myElement = document.querySelector('a');

// Retrieve all computed styles with computedStyleMap()
const styleMap = myElement.computedStyleMap();

// get the <dl> we'll be populating
const stylesList = document.querySelector('#results');

// array of properties we're interested in
const ofInterest = ['font-weight', 'border-left-color', 'color', '--color'];

// iterate over our properties of interest
for ( let i = 0; i < ofInterest.length; i++ ) {

  // properties
  const cssProperty = document.createElement('dt');
  cssProperty.innerText = ofInterest[i];
  stylesList.appendChild(cssProperty);

  // values
  const cssValue = document.createElement('dd');
  // use get() to find the value
  cssValue.innerText = styleMap.get(ofInterest[i]);
  stylesList.appendChild(cssValue);
}