isValidDate in JavaScript

isValidDate.js

export const DAYS_IN_MONTH = [null, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]

function daysInMonth(year, month) {
    // isValidDate checked that year and month are integers already.

    // February of leap years. Assumes that the Gregorian calendar extends
    // infinitely in the future and in the past.
    if (month === 2 && (year % 4 === 0 && (year % 100 !== 0 || year % 400 === 0))) {
        return 29
    }

    // Everything else.
    return DAYS_IN_MONTH[month]
}

export function isValidDate(year, month, day) {
    return (
        // Check that year, month and day are integers. Deals with NaN.
        year === Math.round(year) && month === Math.round(month) && day === Math.round(day) &&
        // Any year is valid. Check that month and day are valid.
        month >= 1 && month <= 12 && day >= 1 && day <= daysInMonth(year, month)
    )
}