How to get a DOM element's ::before content with JavaScript?

I want to know whether it is possible to get a DOM element's ::before content, which was set by CSS3.

I have tried some ways, but I still can't make it, so it's very confusing to me!

// https://rollbar.com/docs/

const links = document.querySelectorAll(`ul.image-list a`);

links[0];
// <a href="/docs/notifier/rollbar-gem/" class="ruby">::before Ruby</a>

links[0];
//

links[0].textContent;
//"Ruby"

links[0].innerText;
// "Ruby"

links[0].innerHTML;
// "Ruby"

// ??? links[0]::before;

This is the case: