HTMLImageElement: width プロパティ

<p>画像の幅: <span class="size">?</span>px (リサイズで更新)</p>
<img
  src="/en-US/docs/Web/HTML/Element/img/clock-demo-200px.png"
  alt="Clock"
  srcset="
    /en-US/docs/Web/HTML/Element/img/clock-demo-200px.png 200w,
    /en-US/docs/Web/HTML/Element/img/clock-demo-400px.png 400w
  "
  sizes="(max-width: 400px) 200px, 400px" />

const clockImage = document.querySelector("img");
let output = document.querySelector(".size");
const updateWidth = (event) => {
  output.innerText = clockImage.width;
};
window.addEventListener("load", updateWidth);
window.addEventListener("resize", updateWidth);