// Month in JavaScript is 0-indexed (January is 0, February is 1, etc), 
// but by using 0 as the day it will give us the last day of the prior
// month. So passing in 1 as the month number will return the last day
// of January, not February
function daysInMonth (month, year) {
    return new Date(year, month, 0).getDate();
}

// July
daysInMonth(7,2009); // 31
// February
daysInMonth(2,2009); // 28
daysInMonth(2,2008); // 29

下面的函数接受任何有效的datetime值，并返回相关月份中的天数…它消除了其他两个答案的模糊性……

 // pass in any date as parameter anyDateInMonth
function daysInMonth(anyDateInMonth) {
    return new Date(anyDateInMonth.getFullYear(), 
                    anyDateInMonth.getMonth()+1, 
                    0).getDate();}

Date.prototype.monthDays= function(){
    var d= new Date(this.getFullYear(), this.getMonth()+1, 0);
    return d.getDate();
}

另一个可能的选择是使用Datejs

然后你就可以

Date.getDaysInMonth(2009, 9)     

虽然仅仅为这个函数添加一个库是多余的，但知道所有可用的选项总是很好的:)