经常会有一些在用户离开页面前执行一些业务的应用场景，这都要用到 onbeforeunload 事件；比如记录用户停留时长的业务，在 GA 等页面访问统计的应用中都包含这个：

;(function(){
    var startTime = Math.ceil(new Date().getTime()/1000), //单位秒
        getDuration = function(){
            var time = '',
                hours = 0,
                minutes = 0,
                seconds = 0,
                endTime = Math.ceil(new Date().getTime()/1000),
                duration = endTime - startTime;

            hours = Math.floor(duration/3600); //停留小时数
            minutes = Math.floor(duration%3600/60); //停留分钟数
            seconds = Math.floor(duration%3600%60); //停留秒数

            time = (hours < 10 ? '0' + hours : hours) + ':' + (minutes < 10 ? '0' + minutes : minutes) + ':' + (seconds < 10 ? '0' + seconds : seconds);

            return time;
        };        


    window.onbeforeunload = function(e){
        var duration = getDuration();

        //request(duration);
    };
})();