使用Javascript计算某年某月有多少天

使用Javascript计算某年的某月共有多少天，程序不是很复杂，只不过要考虑闰年的问题。

          function dayCount(y, m) {      
             var yy = parseInt(y,10);
             var mm = parseInt(m, 10);
             var result = 0;

             if (mm == 1 || mm == 3 || mm == 5 || mm == 7 || mm == 8 || mm == 10 || mm == 12) {
                 result = 31;
             }
             else if (mm == 4 || mm == 6 || mm == 9 || mm == 11) {
                 result = 30;
             } else if (mm == 2) {
                 if ((yy % 4 == 0 && yy % 100 != 0) || yy % 400 == 0) {
                     result = 29;
                 } else {
                     result = 28;
                 }

             }
             return result;
         }