js 输出某年某月某日的天数/判断闰年

   console.log(getDays(2017,12,12));
    function getDays(year,month,day){
        var arr = [31,28,31,30,31,30,31,31,30,31,30,31];
        for(var i=0;i<month-1;i++){
            day += arr[i];
        }
        if(month>2 && isRN(year)){
            day+=1;
        }
        return day;
    }
    function isRN(year){
        if(year%4===0 && year%100!==0 || year%400===0){
            return true;
        }
        return false;
    }