JavaScript获取每个月的天数

function getMonthDay(year, month) {
   let days = new Date(year, month, 0).getDate()
   return days
}
getMonthDay(2021, 3)//30

function getMonthDay(year, month) {
   let days = new Date(year, month, 0).setDate()
   return days
}getMonthDay(2021, 3)//30

function getMonthDay(year) {
   if(year%4==0&&year%100!=0||year%400==0)return 29
   return 28
} 
getMonthDay(2021)//28
getMonthDay(2020)//29