问使用firebase获取$add的角度应用程序不是一个函数

EN

(function(){
    "use strict";

    angular
        .module('todoapp')
        .factory('ListService', ListService);
    ListService.$inject = ['$firebaseArray'];

    function ListService($firebaseArray) {
        var ref = new Firebase('https://---firebase-name---.firebaseio.com/items/'),
        list = $firebaseArray(ref);

        return list;


    }

})();

(function () {
"use strict";

angular
    .module('todoapp')
    .controller('ListController', ListController);

ListController.$inject = ['$scope', '$log', 'ListService'];

/* @ngInject */
function ListController($scope, $log, ListService) {

    var listCtrl = this;

    listCtrl.items = ListService;

    listCtrl.add = function(form){

        //ADD TO FIREBASE
        console.log(listCtrl.newList);
        listCtrl.newList.$add(listCtrl.newList);
}

listCtrl.newList.$add is not a function

list = $firebaseArray(ref).$asArray();