js加载前显示loading（页面加载前）

  工作中遇到，需要加loading。1、用jquery的load方法读取页面，页面读取完成前显示loading。2、首页js等加载过慢，页面加载完成前显示loading。        (1).jquery .load()方法，load完成前显示loading    <!--main_loading的html--><div id="main_loading"  style="display:block;position:fixed;top:0;left:0;width:100%;height:100%;background:#f8f8f8;opacity:0.6;z-index:1000;">        <div id="loading_gif" style="text-align:center;position:fixed;z-index: 9999;top:50%;left: 50%;">                <div class="sk-spinner sk-spinner-cube-grid">                        <div class="sk-cube"></div>                        <div class="sk-cube"></div>                        <div class="sk-cube"></div>                        <div class="sk-cube"></div>                        <div class="sk-cube"></div>                        <div class="sk-cube"></div>                        <div class="sk-cube"></div>                        <div class="sk-cube"></div>                        <div class="sk-cube"></div>                </div>        </div></div><!--js，点击后显示div，加载完成隐藏-->       $("#menu1").click(function(){                        $('#main_loading').show();                        $('#main-page').load('page_a.html',function () {                                $('#main_loading').hide();                        });                })            2、首页页面加载完成前显示loading        <!--main_loading默认显示-->         // 首页加载时loading        document.onreadystatechange = completeLoading;        // 加载状态为complete时移除loading效果        function completeLoading() {                if (document.readyState =="complete") {                        document.getElementById('main_loading').style.display="none";                }                else{                        document.getElementById('main_loading').style.display="block";                }        }