DOMTokenList supports() example

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");
    }
  }
};

var linkSupportsPreload = DOMTokenListSupports(document.createElement("link").relList, "preload"); // true
var classListSupportsFoobar = DOMTokenListSupports(document.createElement("div").classList, "foobar"); // undefined + console.log
var linkSupportsFoobar = DOMTokenListSupports(document.createElement("link").relList, "foobar"); // false

Is this guaranteed to work? Works on Chrome but isn't it possible some browser may implement preload without having tokenList or tokenList.support. If that's the case, then a preload resource may get inserted twice. Once because of onload="..." and once due to JS which doesn't detect preload and inserts the link.