var getDaysInMonth = function(month,year) {  
  // Here January is 1 based  
  //Day 0 is the last day in the previous month  
 return new Date(year, month, 0).getDate();  
// Here January is 0 based  
// return new Date(year, month+1, 0).getDate();  
}  
console.log(getDaysInMonth(1, 2012));  
console.log(getDaysInMonth(2, 2012));  
console.log(getDaysInMonth(9, 2012));  
console.log(getDaysInMonth(12, 2012));  