js X年X周 转成 具体日期

function getWeekDate(theyear,weekcount)
{
var year = theyear;
var week = weekcount;
if(year=="" || week=="") return;

var d = new Date(year, 0, 1);
d.setDate(parseInt("1065432".charAt(d.getDay())) + week * 7);
var fe = getFirstAndEnd(d);
return fe.first.format("MM月dd日")+"-"+ fe.end.format("MM月dd日");
}




Date.prototype.getWeek = function(flag)
{
var first = new Date(this.getFullYear(), 0, 1);
var n = parseInt("1065432".charAt(first.getDay()));
n = this.getTime()-first.getTime()-n*24*60*60*1000;
n = Math.ceil(n/(7*24*60*60*1000));
return (flag==true&&first.getDay()!=1)?(n+1):n;
};
Date.prototype.format = function(format)
{
var o = {
"M+" : this.getMonth()+1, //month
"d+" : this.getDate(), //day
"h+" : this.getHours(), //hour
"m+" : this.getMinutes(), //minute
"s+" : this.getSeconds(), //second
"q+" : Math.floor((this.getMonth()+3)/3), //quarter
"S" : this.getMilliseconds() //millisecond
}
if(/(y+)/.test(format)) format=format.replace(RegExp.$1,
(this.getFullYear()+"").substr(4 - RegExp.$1.length));
for(var k in o)if(new RegExp("("+ k +")").test(format))
format = format.replace(RegExp.$1,
RegExp.$1.length==1 ? o[k] :
("00"+ o[k]).substr((""+ o[k]).length));
return format;
};

function getFirstAndEnd(d)
{
var w = d.getDay(), n = 24*60*60*1000;
var first = new Date(d.getTime() - parseInt("6012345".charAt(w))*n);
var end = new Date(d.getTime() + parseInt("0654321".charAt(w))*n);
return {first: first, end: end};
}

function addDateText(timeStr){
var timeStr_ = timeStr.replace(/年|周/g,",");
var timeStrs = timeStr_.split(",");
console.log(timeStr+"("+getWeekDate(timeStrs[0],timeStrs[1])+")");
}

addDateText("2016年51周"); //显示为 2016年51周(12月19日-12月25日)