gecko-dev/dom/serviceworkers/test/file_blob_response_worker.js

function makeFileBlob(obj) {
  return new Promise(function(resolve, reject) {
    var request = indexedDB.open("file_blob_response_worker", 1);
    request.onerror = reject;
    request.onupgradeneeded = function(evt) {
      var db = evt.target.result;
      db.onerror = reject;

      var objectStore = db.createObjectStore("test", { autoIncrement: true });
      var index = objectStore.createIndex("test", "index");
    };

    request.onsuccess = function(evt) {
      var db = evt.target.result;
      db.onerror = reject;

      var blob = new Blob([JSON.stringify(obj)], { type: "application/json" });
      var data = { blob: blob, index: 5 };

      objectStore = db.transaction("test", "readwrite").objectStore("test");
      objectStore.add(data).onsuccess = function(event) {
        var key = event.target.result;
        objectStore = db.transaction("test").objectStore("test");
        objectStore.get(key).onsuccess = function(event1) {
          resolve(event1.target.result.blob);
        };
      };
    };
  });
}

self.addEventListener("fetch", function(evt) {
  var result = { value: "success" };
  evt.respondWith(
    makeFileBlob(result).then(function(blob) {
      return new Response(blob);
    })
  );
});