Window: getDefaultComputedStyle() メソッド

getDefaultComputedStyle(element)
getDefaultComputedStyle(element, pseudoElt)

const elem = document.getElementById("elemId");
const style = getDefaultComputedStyle(elem);

<div id="elem-container">ダミー</div>
<div id="output"></div>

#elem-container {
  position: absolute;
  left: 100px;
  top: 200px;
  height: 100px;
}

const elem = document.getElementById("elem-container");
const theCSSprop = getDefaultComputedStyle(elem).position;
document.getElementById("output").textContent = theCSSprop; // "static" と出力

<h3>生成コンテンツ</h3>

h3::after {
  content: " rocks!";
}

const h3 = document.querySelector("h3");
const result = getDefaultComputedStyle(h3, "::after").content;
console.log("the generated content is: ", result); // returns 'none'