document.addEventListener('DOMContentLoaded', function() {    //动态创建回到首页dom    var backDom = document.createElement("div");    backDom.className = "z-back";    backDom.innerHTML = '
      
       
      ';    document.body.appendChild(backDom);    //回到首页拖拽效果    //获取屏幕宽高    var screenW = window.screen.availWidth;    var screenH = window.screen.availHeight;    var dragBox = document.querySelector(".z-back");    dragBox.addEventListener("touchstart", function(e) {        var touch = e.touches[0];        startX = touch.pageX;        startY = touch.pageY;        var left = getCss(dragBox, "left");        var top = getCss(dragBox, "top");        var width = getCss(dragBox, "width");        var height = getCss(dragBox, "height");        dragBox.addEventListener("touchmove", function(e) {            e.preventDefault();            var touch = e.touches[0];            var x = touch.pageX - startX;            var y = touch.pageY - startY;            var nowLeft = left + x;            var nowTop = top + y;            //边界值处理            if(x > 0){                nowLeft = nowLeft > screenW - width ? screenW - width : nowLeft;            }else{                nowLeft = nowLeft > 0 ? nowLeft : 0;            }            if(y > 0){                nowTop = nowTop > screenH - height ? screenH - height : nowTop;            }else{                nowTop = nowTop > 0 ? nowTop : 0;            }            dragBox.style.left = nowLeft + "px";            dragBox.style.top = nowTop + "px";        }, false);    }, false);    dragBox.addEventListener("touchend", function(e) {        dragBox.removeEventListener("touchstart,touchmove,touchend", function() {        });    }, false)}, false);//获取元素样式function getCss(curEle, attr) {    var val = null,        reg = null;    if ("getComputedStyle" in window) {        val = window.getComputedStyle(curEle, null)[attr];    } else {        if (attr === "opacity") {            val = curEle.currentStyle["filter"];            reg = /^alpha\(opacity=(\d+(?:\.\d+)?)\)$/i;            val = reg.test(val) ? reg.exec(val)[1] / 100 : 1;        } else {            val = curEle.currentStyle[attr];        }    }    reg = /^(-?\d+(\.\d+)?)(px|pt|rem|em)?$/i;    return reg.test(val) ? parseFloat(val) : val;}