创建上一个功能以查看日历中的上个月

Question

I'm trying to create a calendar, with function of prev and next, As of now the calendar is working find It now display the current month and year. The next function is also working except for the prev function which display the previous month. No problem in next function, but on the prev function it does not produce the correct month after reaching, 0. I suspected that it does not meet the correct condition or the decrement produced negative or something? any idea on this?

  var d = new Date();
  var m = d.getMonth(); 
  var y = d.getFullYear(); 
$(document).on('click', '#next', function(){
          var str = ++m;
          var str1 = y;
          if(str > 11)
          {
              m = 0;
              str1 = y++;
          }
        /* some more codes */ 
  });

$(document).on('click', '#prev', function(){             
          m -= 1;
          var str = m;
          var str1 = y;              
          if(str < 1)
          {
              m = 11;
              str1 = --y;
          }    
         /* some more codes */  
          var url = calendar_vars.plugin_url + "?month=" + str +"&"+"year="+str1;
          xmlhttp.open("GET",url,true);
          xmlhttp.send(); 
  });

Answer 1

Try this :

$(document).on('click', '#next', function(){
   m++;
   if(m > 11) {
      m = 0;
      y++;
   }
   /* some more codes */ 
});
$(document).on('click', '#prev', function(){
          m--;             
          if(m < 0)
          {
              m = 11;
              y--;
          }    
         /* some more codes */  
          var url = calendar_vars.plugin_url + "?month=" + m +"&"+"year="+y;
          xmlhttp.open("GET",url,true);
          xmlhttp.send(); 
    });