原生js封装的获取某一天是当年的第几周方法

           function getWeek(str){
            //str格式为yyy-mm-dd
            //周日归到了本周
             var d=new Date(str);
             var day=d.getDay();
             var originDate=d.getFullYear()+'-'+"01"+'-'+"01"+' 00:00:00';
             var nowDate=d.getFullYear()+'-'+((d.getMonth()+1)>9?(d.getMonth()+1):'0'+(d.getMonth()+1))+"-"+(d.getDate()>9?d.getDate():'0'+d.getDate())+' 00:00:00';
             var nowStrapTime=(new Date(nowDate)).getTime();
             var originStrapTime=(new Date(originDate)).getTime();
             var originDay=((new Date(originStrapTime)).getDay())==0?7:((new Date(originStrapTime)).getDay());
             var nowDay=d.getDay()==0?7:d.getDay();
             var weeks=0;
             var resultStrap=0;
             if(nowDay>originDay){
             	 resultStrap=nowStrapTime-(nowDay-originDay)*1000*3600*24-originStrapTime;
             	if(resultStrap>0){
             		weeks=(resultStrap/(3600*1000*24*7))+1;
             	}else{
             		weeks=1;
             	}
             }else{
             	resultStrap=nowStrapTime+(originDay-nowDay)*1000*3600*24-originStrapTime;
             	weeks=(resultStrap/(3600*1000*24*7))+1;
             }
             return (d.getFullYear()+"年第"+weeks+"周");
           }
           console.log(getWeek('2018-10-31'));//2018年第44周