DOMTokenList supports() example for Preload

domtokenlist_feature_detection.js

var DOMTokenListSupports = function(tokenList, token) {
  if (!tokenList || !tokenList.supports) {
    return;
  }
  try {
    return tokenList.supports(token);
  } catch (e) {
    if (e instanceof TypeError) {
      console.log("The DOMTokenList doesn't have a supported tokens list");
    } else {
      console.error("That shouldn't have happened");
    }
  }
};

// test if preload is supported
var linkSupportsPreload = DOMTokenListSupports(document.createElement("link").relList, "preload"); 
if (linkSupportsPreload) {
  // ...
}