code: function once(emitter, eventName, handler, context) {
    handler = context ? handler.bind(context) : handler;
    let wrapper = function onceHandler(event) {
      emitter.removeEventListener(eventName, onceHandler);
      handler(event);
    };
    emitter.addEventListener(eventName, wrapper);
    return wrapper;
  }
docs: Register to receive one single event.