how to account for leap year in date ago javascript code example

Example: find leap year javascript

function isLeap(year) {
   if (year % 4 === 0) {
      if (year % 100 === 0){
         if (year % 400 == 0){
            return ("Leap year.");
         } else {
            return ("Not leap year.");
         }
      } else {
         return ("Leap year.");
      }
   } else{
      return ("Not leap year.");
   }
}