判断是否是pc，获取屏幕宽度

$(function(){
var w=document.documentElement?document.documentElement.clientWidth:document.body.clientWidth;//先获取窗口宽度
    if(w>=768){
        var J_loginHeight=$(document).height()-$('.J_partfw').outerHeight();
        $('.J_login').css('min-height',J_loginHeight);
    }

    if (isPC()) {
        var contentHeight = document.body.scrollHeight;

        var winHeight;
        if (window.innerHeight != undefined) {
            winHeight = window.innerHeight;
        } else if (document.documentElement.clientHeight != 0) {
            //IE 8
            winHeight = document.documentElement.clientHeight;
        } else {
            //IE6,7
            winHeight = document.body.clientHeight;
        }
        /*if (!(contentHeight > winHeight)) {
            $(".J_partfw").addClass("fixed");
        } else {
            $(".J_partfw").removeClass("fixed");
        }*/
    }

})

function isPC() {
    var userAgentInfo = navigator.userAgent;
    var Agents = ["Android", "iPhone",
        "SymbianOS", "Windows Phone",
        "iPad", "iPod"];
    var flag = true;
    for (var v = 0; v < Agents.length; v++) {
        if (userAgentInfo.indexOf(Agents[v]) > 0) {
            flag = false;
            break;
        }
    }
    return flag;
}