JavaScript 自增自减操作符

<html>
   <body>
      <script>
         var a = 5;
         var b = ++a; var c = a++;
         var d = ++c;
         document.write(a);
         document.write("\r"+b);
         document.write("\r"+c);
         document.write("\r"+d);
      </script>
   </body>
</html>