JavaScript Date.getWeek()? [duplicate]

Answer 1

/*get the week number by following the norms of ISO 8601*/
function getWeek(dt){
	var calc=function(o){
		if(o.dtmin.getDay()!=1){
			if(o.dtmin.getDay()<=4 && o.dtmin.getDay()!=0)o.w+=1;
			o.dtmin.setDate((o.dtmin.getDay()==0)? 2 : 1+(7-o.dtmin.getDay())+1);
		}
		o.w+=Math.ceil((((o.dtmax.getTime()-o.dtmin.getTime())/(24*60*60*1000))+1)/7);
	},getNbDaysInAMonth=function(year,month){
		var nbdays=31;
		for(var i=0;i<=3;i++){
			nbdays=nbdays-i;
			if((dtInst=new Date(year,month-1,nbdays)) && dtInst.getDate()==nbdays && (dtInst.getMonth()+1)==month  && dtInst.getFullYear()==year)
				break;
		}
		return nbdays;
	};
	if(dt.getMonth()+1==1 && dt.getDate()>=1 && dt.getDate()<=3 && (dt.getDay()>=5 || dt.getDay()==0)){
		var pyData={"dtmin":new Date(dt.getFullYear()-1,0,1,0,0,0,0),"dtmax":new Date(dt.getFullYear()-1,11,getNbDaysInAMonth(dt.getFullYear()-1,12),0,0,0,0),"w":0};
		calc(pyData);
		return pyData.w;
	}else{
		var ayData={"dtmin":new Date(dt.getFullYear(),0,1,0,0,0,0),"dtmax":new Date(dt.getFullYear(),dt.getMonth(),dt.getDate(),0,0,0,0),"w":0},
			nd12m=getNbDaysInAMonth(dt.getFullYear(),12);
		if(dt.getMonth()==12 && dt.getDay()!=0 && dt.getDay()<=3 && nd12m-dt.getDate()<=3-dt.getDay())ayData.w=1;else calc(ayData);
		return ayData.w;
	}
}
alert(getWeek(new Date(2017,01-1,01)));