Method preconnectPolyfill_

src/preconnect.js:282–317

* Safari does not support preconnecting, but due to its significant * performance benefits we implement this crude polyfill. * * We make an image connection to a "well-known" file on the origin adding * a random query string to bust the cache (no caching because we do want to * actual

(ampdoc, origin)

   * @private
   */
  preconnectPolyfill_(ampdoc, origin) {
    // Unfortunately there is no reliable way to feature detect whether
    // preconnect is supported, so we do this only in Safari, which is
    // the most important browser without support for it.
    if (
      this.features_.preconnect ||
      !(this.platform_.isSafari() || this.platform_.isIos())
    ) {
      return;
    }

    // Don't attempt to preconnect for ACTIVE_CONNECTION_TIMEOUT_MS since
    // we effectively create an active connection.
    // TODO(@cramforce): Confirm actual http2 timeout in Safari.
    const now = Date.now();
    this.origins_[origin] = now + ACTIVE_CONNECTION_TIMEOUT_MS;
    // Make the URL change whenever we want to make a new request,
    // but make it stay stable in between. While a given page
    // would not actually make a new request, another page might
    // and with this it has the same URL. If (and that is a big if)
    // the server responds with a cacheable response, this reduces
    // requests we make. More importantly, though, it reduces URL
    // entropy as seen by servers and thus allows reverse proxies
    // (read CDNs) to respond more efficiently.
    const cacheBust = now - (now % ACTIVE_CONNECTION_TIMEOUT_MS);
    const url =
      origin +
      '/robots.txt?_AMP_safari_preconnect_polyfill_cachebust=' +
      cacheBust;
    const xhr = new XMLHttpRequest();
    xhr.open('HEAD', url, true);
    // We only support credentialed preconnect for now.
    xhr.withCredentials = true;

    xhr.send();
  }
}

/**