Use

import domData from "can-dom-data";

const element = document.createElement("p");
document.body.appendChild(element);

domData.set(element, "metadata", {
  hello: "world"
});

let metadata = domData.get(element, "metadata");
// metadata is {hello: "world"}

document.body.removeChild(element);

metadata = domData.get(element, "metadata");
// metadata === undefined because the element was removed from the DOM