Date format for javascript

dateformat.js

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;
}

var d = new Date();
d.format("YYYY-MM-dd");