[]"Background is not defined"怎么会没定义呢？？？求救

var Game = (function(){

    (function Game(){

        Laya.init(600,800);

        this.bg = new window.Background();

        Laya.stage.addChild(this.bg);

    })();

})();

var Background = (function(_super){

    function Background(){

        Background.super(this);

        this.bg1 = new Laya.Sprite();

        this.bg1.loadImage("res/background.png");

        this.addChild(this.bg1);

    }

    Laya.class(Background,"Background",_super);

   

    window.Background=Background;

    return Background;

})(Laya.Sprite);