HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>credentialed fetch pattern</title>
  <style>body{font-family:system-ui,sans-serif;margin:1rem;}button{padding:0.5rem 1rem;margin-right:0.5rem;}</style>
</head>
<body>
  <button type="button" id="ask">1) requestStorageAccessFor</button>
  <button type="button" id="fetch">2) credentialed fetch</button>
  <pre id="out">Ready</pre>
  <script>
    var out = document.getElementById("out");
    document.getElementById("ask").addEventListener("click", async function () {
      if (!("requestStorageAccessFor" in document)) {
        out.textContent = "unsupported";
        return;
      }
      try {
        await document.requestStorageAccessFor("https://example.com");
        out.textContent = "granted — try credentialed fetch next";
      } catch (err) {
        out.textContent = "denied: " + (err && err.name ? err.name : err);
      }
    });
    document.getElementById("fetch").addEventListener("click", function () {
      fetch("https://example.com/getcookies.json", {
        method: "GET",
        credentials: "include"
      })
        .then(function (response) { return response.json(); })
        .then(function (json) {
          out.textContent = "got json: " + JSON.stringify(json);
        })
        .catch(function (err) {
          out.textContent = "fetch failed: " + (err && err.message ? err.message : err);
        });
    });
  </script>
</body>
</html>

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Credentialed fetch after storage access (Try It)