遮罩层

.shadow{

    top:0px;
    left:0px;
    right:0px;
    bottom:0px;
    position:absolute;
    display: none;
    background-color: rgba(100,100,100,0.4);
}
// 遮罩层

function shadowB(){

    $(".shadow").css("height",$(document).height());
    $(".shadow").css("width",$(document).width());
    $(".shadow").css('display','block');
}
//隐藏遮罩层

function hideB(){

    $(".shadow").hide();
}