关于JS封装调用的问题

Question

 FivePage = {};
FivePage.show = function() {
    this.change11 = null;
    this.length = 0;
}
FivePage.show.prototype.Ypage1 = function(id) {
    change11 = setInterval(function() {
        length++;
        if(length == $("#" + id + " li").length) {
            length = 0;
        }
        console.log("第五页定时器");
        console.log(this.length);
        console.log(this.ll);
        showaa(length);
    }, 2000);
    function showaa(length) {
        $("#" + id + " li").eq(length - 1).fadeOut(1000); //将上一张图片隐藏
        $("#" + id + " li").eq(length).fadeIn(1000); //将这张图片出现
    }
}
var cbb = new FivePage.show();
btn1.onclick = function() {
    test11.style.display = "block";
    test12.style.display = "none";
    clearInterval(change11);
    alert("现在的下标是："+length+"page1");
    cbb.Ypage1("test11");
}
btn2.onclick = function() {
    test12.style.display = "block";
    test11.style.display = "none";
    clearInterval(change11);
    alert("现在的下标是："+length+"Page2");
    cbb.Ypage1('test12');
};

各位大哥，我现在的问题是，我点击btn2的时候，他的length不会从0开始，我需求是
点击bun1length从0开始走，这个时候，btn1的length走到了3，我在点击btn2的时候，length从0开始走。（这个时候，length会等于3，而不是0），所以问题是他们公用了一个length，我写两个变量，会报错为 not defined
当我再次点击btn1的时候，length会从之前的3继续开始走

Answer 1

    FivePage = {};
    FivePage.show = function () {
        this.change11 = null;
        this.length = 0;
    }
    FivePage.show.prototype.Ypage1 = function (id) {
        var me = this;////闭包，要不setInterval执行的函数中this为window，用me来访问length
        this.change11 = setInterval(function () {
            me.length++;
            if (me.length == $("#" + id + " li").length) {
                me.length = 0;
            }
            console.log("第五页定时器");
            console.log(me.length);
            console.log(me.ll);
            showaa(length);
        }, 2000);
        function showaa(length) {
            $("#" + id + " li").eq(length - 1).fadeOut(1000); //将上一张图片隐藏
            $("#" + id + " li").eq(length).fadeIn(1000); //将这张图片出现
        }
    }
    var cbb = new FivePage.show();
    btn1.onclick = function () {
        test11.style.display = "block";
        test12.style.display = "none";
        clearInterval(cbb.change11);///////////////////////
        alert("现在的下标是：" + length + "page1");
        cbb.Ypage1("test11");
    }
    btn2.onclick = function () {
        test12.style.display = "block";
        test11.style.display = "none";
        clearInterval(cbb.change11);////////////////
        alert("现在的下标是：" + length + "Page2");
        cbb.Ypage1('test12');
    };