Observe size change of element

This is not a duplicate! I’m not satisfied with the result of other answers. If you still think it is a duplicate, please comment it before why you think it is!

I have to observe size changes of elements. I’ve tried to do it by creating a setter on the offsetWidth property like this:

const div = document.querySelector("div");

div.addEventListener("resize", console.log);
div.addEventListener("click", () => console.log(div.offsetWidth));

let width = div.offsetWidth;
Object.defineProperty(div, "offsetWidth", {
    get: () => width,
  set: val => {
    width = val;
    div.dispatchEvent(new Event("resize"));

document.querySelector("button").onclick = () => div.style.width = "200px";
div {
  background-color: #f00;
  width: 300px;
  height: 300px;
Set size

Click on the red box to log the size. Then press the button. Nothing happens. If you click the box again, it will still show the old size. Why does this not work?

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