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

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

    options = L.setOptions(this, options);

    this._layers = {};
    this._leafletIds = {};

    this.cluster = L.markerClusterGroup(options);
    this._key = 'c' + (Math.random() * 1e9).toString(36).replace('.', '_');

    this.cluster.addEventParent(this);
  },

onRemove: function (map) {
    FeatureManager.prototype.onRemove.call(this, map);
    this._map.removeLayer(this.heat);
  },

onAdd: function (map) {
    FeatureManager.prototype.onAdd.call(this, map);
    this._map.addLayer(this.cluster);
  },

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

    options = L.setOptions(this, options);

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

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