dynamic-resource-hints/index.js

Dynamically add preload browser hints at runtime

var makeHint = function makeHint(href, rel, as, crossorigin, type) {
  var link = document.createElement("link");
  var existingLink = document.querySelector("link[href=\"" + href + "\"]");

  if (existingLink) {
    if (existingLink.getAttribute(rel)) return;
  }

  link.setAttribute("rel", rel);
  link.setAttribute("href", href);
  crossorigin && link.setAttribute("crossorigin", crossorigin);
  type && link.setAttribute("type", type);
  as && link.setAttribute("as", as);
  document.head.appendChild(link);
  return href;
};

var hasBeenPrerendered = [];
var hasBeenPrefetched = [];
var allowedRels = ['preload', 'prerender', 'subresource', 'prefetch', 'preconnect', 'dns-prefetch'];

function preloadResource(href, rel, as, crossorigin, type) {
  if (!rel) {
    rel = 'prerender';
  }

  if (!crossorigin) {
    crossorigin = false;
  }

  if (!allowedRels.includes(rel)) {
    throw new Error('preloadResource: you have not entered a valid preload relative. Only the following are allowed: \'preload\',\'prefetch\',\'prerender\',\'subresource\',\'prefetch\',\'preconnect\',\'dns-prefetch\' ');
  }

  if (!href) {
    throw new Error('preloadResource: you must specify an href');
  }

  if (rel === "prefetch" || rel === "preload") {
    if (!as) {
      throw new Error('preloadResource: you must specify what "as" type resource this is when preloading or prefetching. Read - https://medium.com/reloading/preload-prefetch-and-priorities-in-chrome-776165961bbf');
    }
  }

  makeHint(href, rel, as, crossorigin, type);
}

module.exports = preloadResource;