求助关于移动端手指上下滑动松开后，元素会惯性滑动一点距离！！

touchMoveSliderCateList = function(x, y){
        var $list = jqueryMap.$cateListUl,
            //    获取用户手指滑动距离
            offset = {
                x : x - stateMap.touchStartPos.x,
                y : y - stateMap.touchStartPos.y
            };

        $list.css('transform', 'translate3d(0px, ' + (offset.y - Math.abs(stateMap.listOffsetTop)) + 'px, 0px)');
};

目前我是用tuchmove的最后位置减去touchstart的起始位置，得到用户手指滑动的距离，在用户移动的途中不停的设置到元素上，移动元素是没问题，但是感觉太生硬，我希望在我手指松开后，能偶惯性移动一点距离，如何做呢？？

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用手拖动ul，可以根据速度，惯性移动不同距离，超出范围了还能回弹,供参考~

<script type="text/javascript">
        //得到各种元素
        var nav = document.querySelector("nav");
        var navul = document.querySelector("nav ul");
        var navullis = document.querySelectorAll("nav ul li");
        var navW = parseInt(window.getComputedStyle(nav , null)['width']);

        //宽度
        navul.style.width = navullis.length * 82 +"px";

        nav.addEventListener("touchstart",touchstartHandler);
        nav.addEventListener("touchmove",touchmoveHandler);
        nav.addEventListener("touchend",touchendHandler);

        var startX;
        var nowX = 0;
        var dX;

        var lastTwoPoint = [0,0];

        //开始滑动
        function touchstartHandler(event){
            navul.style.webkitTransition = "none";    //去掉过渡
            navul.style.transition = "none";    //去掉过渡
            startX = event.touches[0].pageX;    //记录起点
        }

        //滑动过程
        function touchmoveHandler(event){
            event.preventDefault();
            dX = event.touches[0].pageX - startX;    //差值

            //反映差值
            navul.style.webkitTransform = "translateX(" + (nowX + dX) + "px)";
            navul.style.transform = "translateX(" + (nowX + dX) + "px)";

            //记录最后两点的x值
            lastTwoPoint.shift();
            lastTwoPoint.push(event.touches[0].pageX);
        }

        //结束滑动
        function touchendHandler(event){
            nowX += dX;

            //多走最后两点路程的5倍路程
            nowX += (lastTwoPoint[1] - lastTwoPoint[0]) * 5;
            if(nowX > 0){
                nowX = 0;
            }

            if(nowX < -parseInt(navul.style.width) + navW){
                nowX = -parseInt(navul.style.width) + navW;
            }

            console.log(-parseInt(navul.style.width) + navW)
            //过渡时间
            //非线性衰减
            var t = Math.sqrt(Math.abs(lastTwoPoint[1] - lastTwoPoint[0])) / 10;

            navul.style.webkitTransition = "all " + t + "s cubic-bezier(0.1, 0.85, 0.25, 1) 0s"; 
            navul.style.transition = "all " + t + "s cubic-bezier(0.1, 0.85, 0.25, 1) 0s"; 

            //反映多走的5倍路程：
            navul.style.webkitTransform = "translateX(" + nowX + "px)";
            navul.style.transform = "translateX(" + nowX + "px)";
        }
    </script>