HTMLLinkElement.relList

The HTMLLinkElement.relList read-only property reflects the rel attribute. It is a live DOMTokenList containing the set of link types indicating the relationship between the resource represented by the <link> element and the current document.

The property itself is read-only, meaning you can not substitute the DOMTokenList by another one, but the content of the returned list can be changed.

const links = document.getElementsByTagName("link");
const length = links.length;
for (let i = 0; i < length; i++) {
  const list = links[i].relList;
  const listLength = list.length;
  console.log("New link found.");
  for (let j = 0; j < listLength; j++) {
    console.log(list[j]);
  }
}