Js 根据年月获取这月或者年的开始日期和结束日期

//获取这个月的月初和月末
function getMonthStartEnd(vars){
    var str = '';
    if(vars!=null&&vars!=''){
        var nyYear=vars.slice(0,4);
        var nyMonth=vars.slice(4,vars.length);
        var firstDay = new Date(nyYear,nyMonth-1);
        var lastDay = new Date(new Date(nyYear,nyMonth).valueOf()-60*60*1000*24);
        function datasFormat(d){
            var datetime=d.getFullYear() + '-' + (d.getMonth() + 1) + '-' + d.getDate();
            return datetime;
        }
        str = datasFormat(firstDay) + "/" + datasFormat(lastDay)
     }
    return str
}

//获取年的开始日期和结束日期
function getYearStartEnd(vars){
     return vars+"-01-01/"+vars+"-12-31"
}