获取某个月的最后一天

 /**
     * 获取某个月的最后一天
     * y:2020
     * m:4
     */
    getLastDay(y, m) {
      let date = new Date(y, m, 1);
      let lastDay = new Date(date.getTime() - 1000 * 60 * 60 * 24);
      return lastDay.getDate();
    },