原生JS输入某年某月某天，判断这天是本年的第几天

function isLpYear(year) {
        // 假设不是闰年
        var isLeapYear = false;
        if ((year % 4 === 0 && year % 100 !== 0) || year % 400 === 0) {
            isLeapYear = true;
        }
        return isLeapYear;
    }

    function getDays(year, month, day) {
        if (year >= 1980 && year <= 2040) {
            var days = day;
            for (var i = 1; i < month; i++) {
                switch (i) {
                    case 1:
                    case 3:
                    case 5:
                    case 7:
                    case 8:
                    case 10:
                    case 12:
                        days = days + 31;
                        break;
                    case 4:
                    case 6:
                    case 9:
                    case 11:
                        days = days + 30;
                        break;
                    case 2:
                        if (isLpYear(year)) {
                            days = days + 29;
                        } else {
                            days = days + 28;
                        }
                        break;
                }
            }
            return days;
        } else {
            document.write("请输入正确的年份");
        }
    }
    var year = document.getElementById("text1");
    var month = document.getElementById("text2");
    var day = document.getElementById("text3");
    var bn = document.getElementById("bn");
    bn.onclick = function() {
        document.write("这是本年的第" + getDays(parseInt(year.value), parseInt(month.value), parseInt(day.value)) + "天");
    };