Validate number of days in a given month

Question

Performance is of the utmost importance on this one guys... This thing needs to be lightning fast!


How would you validate the number of days in a given mont

Answer 1

This will not perform as well as the accepted answer. I threw this in here because I think it is the simplest code. Most people would not need to optimize this function.

function validateDaysInMonth(year, month, day)
{
    if (day < 1 || day > 31 || (new Date(year, month, day)).getMonth() != month)
        throw new Error("Frack!");
}

It takes advantage of the fact that the javascript Date constructor will perform date arithmetic on dates that are out of range, e.g., if you do:

var year = 2001; //not a leap year!
var month = 1 //February
var day = 29; //not a valid date for this year
new Date(year, month, day);

the object will return Mar 1st, 2001 as the date.