在页面加载前先出现加载loading，页面加载完成之后再显示页面

//在页面未加载完毕之前显示的loading Html自定义内容

var _LoadingHtml = ‘页面加载中，请等待...‘;

//呈现loading效果

document.write(_LoadingHtml);

//监听加载状态改变
document.onreadystatechange = completeLoading;

//加载状态为complete时移除loading效果
function completeLoading() {
    if (document.readyState == "complete") {
        var loadingMask = document.getElementById(‘loadingDiv‘);
        loadingMask.parentNode.removeChild(loadingMask);
    }
}

//获取浏览器页面可见高度和宽度
var _PageHeight = document.documentElement.clientHeight,
    _PageWidth = document.documentElement.clientWidth;
//计算loading框距离顶部和左部的距离（loading框的宽度为215px，高度为61px）
var _LoadingTop = _PageHeight > 61 ? (_PageHeight - 61) / 2 : 0,
    _LoadingLeft = _PageWidth > 215 ? (_PageWidth - 215) / 2 : 0;
//在页面未加载完毕之前显示的loading Html自定义内容
var _LoadingHtml = `<div id="loadingDiv" 
style="position:absolute;left:0;width:100%;height:‘
+ _PageHeight + ‘px;top:0;background:#f3f8ff;opacity:1;
filter:alpha(opacity=80);z-index:10000;"><div 
style="position: absolute; cursor1: wait; left: ‘ 
+ _LoadingLeft + ‘px; top:‘ + _LoadingTop + ‘px; 
width: auto; height: 57px; line-height: 57px;
padding-left: 50px; padding-right: 5px; 
background: #fff url(image/dd.png) 
no-repeat scroll 5px 10px; border: 2px solid #95B8E7;
color: #696969; font-family:\‘Microsoft YaHei\‘;">页面加载中，请等待...</div></div>`;
//呈现loading效果
document.write(_LoadingHtml);
   
//window.onload = function () {
//    var loadingMask = document.getElementById(‘loadingDiv‘);
//    loadingMask.parentNode.removeChild(loadingMask);
//};
   
//监听加载状态改变
document.onreadystatechange = completeLoading;
   
//加载状态为complete时移除loading效果
function completeLoading() {
    if (document.readyState == "complete") {
        var loadingMask = document.getElementById(‘loadingDiv‘);
        loadingMask.parentNode.removeChild(loadingMask);
    }
}