myLib.js

function isBissextile(year) {
    if (year % 4 === 0 && year % 100 !== 0) {
        return true;
    }
    if (year % 400 === 0) {
        return true;
    }
    return false;
}


function getMaxDayPerMonth(month, year) {
    if (isNaN(month) || month < 1 || month > 12 || isNaN(year) || year < 1) {
        console.error(`Month must be [1-12]`);
        console.error(`Year must be >0`);
        return -1;
    }
    let maxDays = 31;
    switch (month) {
        case 4:
        case 6:
        case 9:
        case 11:
            maxDays = 30;
            break;
        case 2:
            if (isBissextile(year) === true) {
                maxDays = 29;
            } else {
                maxDays = 28;
            }
    }
    return maxDays;
}

function isValid(day, month, year) {
    if (isNaN(day) || isNaN(month) || isNaN(year) || day <= 0 || month <= 0 || year <= 0 || month > 12) {
        return false;
    }
    return day > 0 && day <= getMaxDayPerMonth(month, year);
}