Retrieves an existing push subscription.
The permissionState() method of the PushManager interface returns a Promise that resolves to a DOMString indicating the permission state of the push manager.
An object containing optional configuration parameters. It can have the following properties: - userVisibleOnly: A boolean indicating that the returned push subscription will only be used for messages whose effect is made visible to the user.
The subscribe() method of the PushManager interface subscribes to a push service.