What is the best way to determine the number of days in a month with JavaScript?

function daysInMonth (month, year) { // Use 1 for January, 2 for February, etc.
  return new Date(year, month, 0).getDate();
}

console.log(daysInMonth(2, 1999)); // February in a non-leap year.
console.log(daysInMonth(2, 2000)); // February in a leap year.

Day 0 is the last day in the previous month. Because the month constructor is 0-based, this works nicely. A bit of a hack, but that's basically what you're doing by subtracting 32.

See more : Number of days in the current month

With moment.js you can use daysInMonth() method:

moment().daysInMonth(); // number of days in the current month
moment("2012-02", "YYYY-MM").daysInMonth() // 29
moment("2012-01", "YYYY-MM").daysInMonth() // 31

Some answers (also on other questions) had leap-year problems or used the Date-object. Although javascript's Date object covers approximately 285616 years (100,000,000 days) on either side of January 1 1970, I was fed up with all kinds of unexpected date inconsistencies across different browsers (most notably year 0 to 99). I was also curious how to calculate it.

So I wrote a simple and above all, small algorithm to calculate the correct (Proleptic Gregorian / Astronomical / ISO 8601:2004 (clause 4.3.2.1), so year 0 exists and is a leap year and negative years are supported) number of day's for a given month and year.
It uses the short-circuit bitmask-modulo leapYear algorithm (slightly modified for js) and common mod-8 month algorithm.

Note that in AD/BC notation, year 0 AD/BC does not exist: instead year 1 BC is the leap-year!
IF you need to account for BC notation then simply subtract one year of the (otherwise positive) year-value first!! (Or subtract the year from 1 for further year-calculations.)

function daysInMonth(m, y){
  return m===2?y&3||!(y%25)&&y&15?28:29:30+(m+(m>>3)&1);
}

<!-- example for the snippet -->
<input type="text" value="enter year" onblur="
  for( var r='', i=0, y=+this.value
     ; 12>i++
     ; r+= 'Month: ' + i + ' has ' + daysInMonth(i, y) + ' days<br>'
     );
  this.nextSibling.innerHTML=r;
" /><div></div>

Note, months must be 1-based!

Note, this is a different algorithm then the magic number lookup I used in my Javascript calculate the day of the year (1 - 366) answer, because here the extra branch for the leap-year is only needed for February.

If you call this function often, it may be useful to cache the value for better performance.

Here is caching version of FlySwat's answer:

var daysInMonth = (function() {
    var cache = {};
    return function(month, year) {
        var entry = year + '-' + month;

        if (cache[entry]) return cache[entry];

        return cache[entry] = new Date(year, month, 0).getDate();
    }
})();