How to use the @jupyter-widgets/base.DOMWidgetModel.prototype function in @jupyter-widgets/base

defaults() {
        return {...DOMWidgetModel.prototype.defaults(), 
            _model_name: "TooltipModel",
            _view_name: "Tooltip",
            _model_module: "bqplot",
            _view_module: "bqplot",
            _model_module_version: semver_range,
            _view_module_version: semver_range,

            fields: [],
            formats: [],
            show_labels: true,
            labels: []
        };
    }
}