function getDateNum(params) {
  let temp = null;
  let getYear = params.getFullYear();
  let getMonth = params.getMonth() + 1;
  // new Date(year, month, 0).getDate()
  temp = new Date(getYear, getMonth, 0).getDate();
  return temp;
}
console.log('结果:', getDateNum(new Date()));

/**
 * 根据日期获取当月天数
 * @param {string|number} month - 月份
 * @param {string|number} [year] - 年份
 * @returns {number} 天数
 */
function daysInMonth(month, year) {
    year = year || new Date().getFullYear();
    return new Date(year, month, 0).getDate();
}

daysInMonth(8, 2019); // => 31
daysInMonth(2, 2020); // => 29
daysInMonth(2); // => 28

const daysInMonth = (date) => new Date(date.getFullYear(), date.getMonth() + 1, 0).getDate();