js算出生日是当年第多少天

var year, month, day, monthSum = 0;
var arr = new Array(31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31);
function getDate() {

    year = Number(prompt("请输入年"));
    month = Number(prompt("请输入你的出生月份"));
    day = Number(prompt("请输入你的出生日期"));

    for (var i = 0; i < month - 1; i++) {
        monthSum = monthSum + arr[i]
    }
    if ((year % 400 == 0 || (year % 4 == 0 && year % 100 != 0)) && month > 2) {
        monthSum = monthSum + day + 1
        document.write("您的生日在" + year + "年" + "是第" + monthSum + "天");
    } else {
        monthSum = monthSum + day
        document.write("您的生日在" + year + "年" + "是第" + monthSum + "天");
    }
}
getDate();