JavaScript function to determine if the specified Date object occurs in a Leap year.

/**
 * Check if the given date occurs in a leap year
 *
 * from date-fns (modern JavaScript date utility library) `isLeapYear`
 * https://github.com/date-fns/date-fns/blob/master/src/isLeapYear/index.js
 *
 * @param {Date} date - the date to check
 * @returns {Boolean} the date is in the leap year
 * @example
 *     // Is 1 September 2012 in the leap year?
 *     var result = isLeapYear(new Date(2012, 8, 1));
 *     //=> true
 */
function isLeapYear(date) {
    var year = date.getFullYear();
    return year % 400 === 0 || (year % 4 === 0 && year % 100 !== 0);
}