Get Pseudo-Element Properties with JavaScript

HTML

<div id="box">aaaaaaa</div>

CSS

#box::after {
	content: 'BIER';
	color: rgb(255, 0, 0);
}

JavaScript

var property = window.getComputedStyle(document.querySelector("#box", '::after')).getPropertyCSSValue("content").cssText;
console.log(property);