notifications.update()

let updating = browser.notifications.update(
  id,                            // string
  options                        // NotificationOptions
)

let cakeNotification = "cake-notification";

/*

CAKE_INTERVAL is set to 0.3 seconds in this example.
Such a short period is chosen to make the extension's behavior
more obvious, but this is not recommended in real life.
Note that in Chrome, alarms cannot be set for less than
a minute.

*/
let CAKE_PREP_INTERVAL = 0.005;

let progress = 0;

browser.alarms.onAlarm.addListener(function(alarm) {
  progress = progress + 10;
  if (progress > 100) {
    browser.notifications.clear(cakeNotification);
    browser.alarms.clear("cake-progress");
  } else {
    browser.notifications.update(cakeNotification, {
      "progress": progress
    });
  }
});

browser.browserAction.onClicked.addListener(function () {
  browser.notifications.getAll((all) => {
    if (all.length > 0) {
      browser.notifications.clear(cakeNotification);
      return;
    }
    progress = 0;
    browser.notifications.create(cakeNotification, {
      "type": "progress",
      "iconUrl": browser.extension.getURL("icons/cake-48.png"),
      "title": "Your cake is being prepared...",
      "message": "Something something cake",
      "progress": progress
    });
    browser.alarms.create(
      "cake-progress",
      {periodInMinutes: CAKE_PREP_INTERVAL}
    );
  });
});