10.获取某个月的最后一天，或者说是计算某个月的天数

/* 获取指定月份的最后一天 */
function getMOnthLastDay(year, month) {
    var d = new Date(0);
    if (12 == month) {
        d.setUTCFullYear(year + 1);
        d.setUTCMonth(0);
    } else {
        d.setUTCFullYear(year);
        d.setMonth(month);
    }
    d.setTime(d.getTime() - 1);
    if (month <= 9) {
        month = "0" + month;
    }
    return year + "-" + month + "-" + d.getUTCDate();
};

console.log(getMOnthLastDay(2018, 2));//结果：2018-02-28