t;, key));
      req.send();
      await loadPromise;
      return key;
    }

read: async (key) =&gt; {
      const req = new XMLHttpRequest();
      const loadPromise = new Promise(resolve =&gt; req.addEventListener(&quot;load&quot;, resolve));
      req.open(&quot;GET&quot;, testURI(&quot;resource&quot;, &quot;xhr&quot;, key));
      req.send();
      await loadPromise;
      let countResponse = await fetch(testURI(&quot;ctr&quot;, &quot;xhr&quot;, key),
                                      {cache: &quo