给document.documentElement.scrollTop赋值以后怎么返回成可变量？

如题，

为了仿照css3中transform 2D平移事件

        <script>
            var a = document.getElementsByClassName('a');
            var i = 0;

            for (i = 0; i < a.length; i++) {
                a[i].onmouseover = function () {
                    startMove3(this);//注意，这里是要做一个触摸按钮然后页面移动到指定位置，其中的变量已经固定。
                }
            }

            //获取目标元素到顶部的高度
            function getElementToPageTop(el) {
                if (el.parentElement) {
                    return this.getElementToPageTop(el.parentElement) + el.offsetTop
                }
                return el.offsetTop
            }

            function startMove3(obj) {
                //获取当前的位置
                clearInterval(obj.timer);
                obj.timer = setInterval(function () {
                    var iCur = 0;

                    iCur = document.documentElement.scrollTop || document.body.scrollTop; //注意检查这里的

                    //*计算目标

                    //页面可见高
                    var iCurBodyHeight = document.body.clientHeight;
                    //盒子的总高
                    var mainBannerHeight = document.getElementById('main-banner').offsetHeight;
                    //移动图片的高，四张图片
                    var oHeight = mainBannerHeight / 4;
                    //需要居中时top
                    var oMargin = (iCurBodyHeight - oHeight) / 2;
                    //目标
                    var iTarget = 0;
                    var iTargetNeeds = 0;

                    //移动盒子的对应至最顶部的高

                    if (obj == a[0]) {
                        iTargetNeeds = getElementToPageTop(document.getElementById('lists1'));
                    } else if (obj == a[1]) {
                        iTargetNeeds = getElementToPageTop(document.getElementById('lists2'));
                    } else if (obj == a[2]) {
                        iTargetNeeds = getElementToPageTop(document.getElementById('lists3'));
                    } else {
                        iTargetNeeds = document.documentElement.offsetHeight;
                    } 


                    //需要被卷走的距离
                    iTarget = iTargetNeeds - oMargin;

                    //计算速度
                    var iSpeed = (iTarget - iCur) / 8;
                    iSpeed = iSpeed > 0 ? Math.ceil(iSpeed) : Math.floor(iSpeed);

                    //检测停止 
                    if (iCur != iTarget) {
                        bStop = false;
                    }

                    document.documentElement.scrollTop = iCur + iSpeed; //执行一次以后成为固定值，这里怎么使其能够改变，

                    if (bStop) {
                        clearInterval(obj.timer);

                        if (fn) {
                            fn();
                        }
                    }
                }, 30)
            }
        </script>

window.addEventListener('scroll',function(){});

具体解答方法：
贴上代码

            var a = document.getElementsByClassName('main-guild-inner-list');
            var i = 0;

            for (i = 0; i < a.length; i++) {
                a[i].style.backgroundColor = 'red';


                a[i].onmouseover = function () {
                    startMove3(this);
                }
                a[i].onmouseout = function() {
                    clearInterval(this.timer)
                }
            }

            //获取目标元素到顶部的高度
            function getElementToPageTop(el) {
                if (el.parentElement) {
                    return this.getElementToPageTop(el.parentElement) + el.offsetTop
                }
                return el.offsetTop
            }

            function startMove3(obj, fn) {
                //获取当前的位置
                clearInterval(obj.timer);
                obj.timer = setInterval(function () {
                    var iCur = 0;

                    iCur = document.documentElement.scrollTop || document.body.scrollTop; //注意检查这里的

                    //*计算目标
                    //页面可见高
                    var iCurBodyHeight = document.body.clientHeight;
                    //盒子的总高
                    var mainBannerHeight = document.getElementById('main-banner').offsetHeight;
                    //移动图片的高
                    var oHeight = mainBannerHeight / 4;
                    //需要居中时top
                    var oMargin = (iCurBodyHeight - oHeight) / 2;
                    //目标
                    var iTarget = 0;
                    var iTargetNeeds = 0;

                    //移动盒子的对应至最顶部的高

                    if (obj == a[0]) {
                        iTargetNeeds = getElementToPageTop(document.getElementById('main-banner-inner-lists1'));
                    } else if (obj == a[1]) {
                        iTargetNeeds = getElementToPageTop(document.getElementById('main-banner-inner-lists2'));
                    } else if (obj == a[2]) {
                        iTargetNeeds = getElementToPageTop(document.getElementById('main-banner-inner-lists3'));
                    } else {
                        iTargetNeeds = document.documentElement.offsetHeight;
                    }


                    //需要被卷走的距离
                    iTarget = iTargetNeeds - oMargin;



                    //计算速度
                    var iSpeed = (iTarget - iCur) / 8;
                    iSpeed = iSpeed > 0 ? Math.ceil(iSpeed) : Math.floor(iSpeed);

                    //检测停止 
                    if (iTarget != 0) {
                        bStop = false;
                    }

                    document.documentElement.scrollTop += iSpeed; //执行一次以后成为固定值，这里怎么使其能够改变，

                    if (bStop) {
                        clearInterval(obj.timer);

                        if (fn) {
                            fn();
                        }
                    }
                }, 30)
            }