code: function watchOnce(paths, callback) {
      var options = arguments.length > 2 && arguments[2] !== undefined ? arguments[2] : { priority: 0 };
  
      var id = void 0;
      var onceCallback = function _watchOnceCallback() {
        unwatch(id);
        callback.apply(undefined, arguments);
      };
  
      id = watch(paths, onceCallback, options);
    }
docs: Watch for changes on a given key once, and execute the provided callback
  when there are changes. Callback removes itself after the first call so
  that it is never called again.
  
  @param {String[]|String} String path or array of paths to watch
  @param {Function} callback to execute when there are changes
  @param {Object} options Options
  @param {Number} options.priority Controls the order the provided callback is called when multiple watches exist on the same key.