How to use the esri-leaflet.FeatureManager.extend function in esri-leaflet

export { version as VERSION } from '../package.json';

import L from 'leaflet';
import { FeatureManager } from 'esri-leaflet';

export var FeatureLayer = FeatureManager.extend({
  /**
   * Constructor
   */

  initialize: function (options) {
    FeatureManager.prototype.initialize.call(this, options);

    options = L.setOptions(this, options);

    this._cache = {};
    this._active = {};

    this.heat = window.L.heatLayer([], options);
  },

  /**

import L from 'leaflet';
import { FeatureManager } from 'esri-leaflet';
export { version as VERSION } from '../package.json';

export var FeatureLayer = FeatureManager.extend({

  statics: {
    EVENTS: 'click dblclick mouseover mouseout mousemove contextmenu popupopen popupclose',
    CLUSTEREVENTS: 'clusterclick clusterdblclick clustermouseover clustermouseout clustermousemove clustercontextmenu'
  },

  /**
   * Constructor
   */

  initialize: function (options) {
    FeatureManager.prototype.initialize.call(this, options);

    options = L.setOptions(this, options);

    this._layers = {};