How can I get final hex color in Javascript from a relative CSS color string?

Let’s say I have a CSS like that, which uses CSS Relative Colors syntax:

.my-element {
  color: rgb(from #aabbcc r + 30 g b - 30 / 80%);
}

(in real life, this example would use CSS variables which I know how to substitute with actual values, so I simplified it).

In CSS it calculates a color with increased red and decreased blue channels, and with 80% alpha-channel.

I need to get that color and further manipulate it in my JS. Tried using getComputedStyle – it doesn’t help, just returns the initial string representation of the color.

So, is there a way to get that final color in JS? Any built-in solution or a library?