How to use the @jupyter-widgets/controls.ButtonModel.prototype function in @jupyter-widgets/controls

// limitations under the License.


import {
  ButtonView,
  ButtonModel
} from '@jupyter-widgets/controls';

import {
  extend
} from 'lodash';


// Custom Model. Custom widgets models must at least provide default values
var ImageButtonModel = ButtonModel.extend({
  defaults: extend(ButtonModel.prototype.defaults(), {
    _model_name: 'ImageButtonModel',
    _view_name: 'ImageButtonView',
    _model_module: 'jupyter_cadquery',
    _view_module: 'jupyter_cadquery',
    _model_module_version: "v0.9.5",
    _view_module_version: "v0.9.5",
    value: null
  })
});


// Custom View. Renders the widget model.
var ImageButtonView = ButtonView.extend({
  render: function () {
    var width = this.model.get('width');
    var height = this.model.get('height');