OpenLayers3, given an initial layer (a group or a single layer) finds recursively a layer with the specified key-value.

gistfile1.js

            /**
             * Finds recursively the layer with the specified key and value.
             * @param {ol.layer.Base} layer
             * @param {String} key
             * @param {any} value
             * @returns {ol.layer.Base}
             */
            function findBy(layer, key, value) {

                if (layer.get(key) === value) {
                    return layer;
                }

                // Find recursively if it is a group
                if (layer.getLayers) {
                    var layers = layer.getLayers().getArray(),
                            len = layers.length, result;
                    for (var i = 0; i < len; i++) {
                        result = findBy(layers[i], key, value);
                        if (result) {
                            return result;
                        }
                    }
                }

                return null;
            }