Leaflet/src/layer/FeatureGroup.js

/*
 * L.FeatureGroup extends L.LayerGroup by introducing mouse events and bindPopup method shared between a group of layers.
 */

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

	addLayer: function (layer) {
		this._initEvents(layer);
		L.LayerGroup.prototype.addLayer.call(this, layer);

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

	bindPopup: function (content) {
		this._popupContent = content;

		for (var i in this._layers) {
			if (this._layers.hasOwnProperty(i) && this._layers[i].bindPopup) {
				this._layers[i].bindPopup(content);
			}
		}
	},

	_events: ['click', 'dblclick', 'mouseover', 'mouseout'],

	_initEvents: function (layer) {
		for (var i = 0, len = this._events.length; i < len; i++) {
			layer.on(this._events[i], this._propagateEvent, this);
		}
	},

	_propagateEvent: function (e) {
		e.layer = e.target;
		e.target = this;
		this.fire(e.type, e);
	}
});