Leaflet/src/layer/FeatureGroup.js

/*
 * L.FeatureGroup extends L.LayerGroup by introducing mouse events and additional methods
 * shared between a group of interactive layers (like vectors or markers).
 */

L.FeatureGroup = L.LayerGroup.extend({
	includes: L.Mixin.Events,

	statics: {
		EVENTS: 'click dblclick mouseover mouseout mousemove contextmenu popupclose'
	},

	addLayer: function (layer) {
		if (this.hasLayer(layer)) {
			return this;
		}

		layer.on(L.FeatureGroup.EVENTS, this._propagateEvent, this);

		L.LayerGroup.prototype.addLayer.call(this, layer);

		if (this._popupContent && layer.bindPopup) {
			layer.bindPopup(this._popupContent, this._popupOptions);
		}

		return this.fire('layeradd', {layer: layer});
	},

	removeLayer: function (layer) {
		if (layer in this._layers) {
			layer = this._layers[layer];
		}

		layer.off(L.FeatureGroup.EVENTS, this._propagateEvent, this);

		L.LayerGroup.prototype.removeLayer.call(this, layer);

		if (this._popupContent) {
			this.invoke('unbindPopup');
		}

		return this.fire('layerremove', {layer: layer});
	},

	bindPopup: function (content, options) {
		this._popupContent = content;
		this._popupOptions = options;
		return this.invoke('bindPopup', content, options);
	},

	setStyle: function (style) {
		return this.invoke('setStyle', style);
	},

	bringToFront: function () {
		return this.invoke('bringToFront');
	},

	bringToBack: function () {
		return this.invoke('bringToBack');
	},

	getBounds: function () {
		var bounds = new L.LatLngBounds();

		this.eachLayer(function (layer) {
			bounds.extend(layer instanceof L.Marker ? layer.getLatLng() : layer.getBounds());
		});

		return bounds;
	},

	_propagateEvent: function (e) {
		if (!e.layer) {
			e.layer = e.target;
		}
		e.target = this;

		this.fire(e.type, e);
	}
});

L.featureGroup = function (layers) {
	return new L.FeatureGroup(layers);
};