javascript面向对象学习 - 输入框最大可输入字符

/* 最大可输入字符 */

     function   watching_limit(option){

              this ._watch = document.getElementById(option.watch); //监听的dom元素

           this   ._tip = document.getElementById(option.tip);              //实时提示_watch_obj的dom

           this   ._limit = option.limit    //限制的字数

           this   ._is_byte = option.is_byte          //一个汉字是否按两个字节来计算

     }

     watching_limit.prototype = {

           get_len:   function (){

                   var   _that =   this ;

                   var   length = 0;

                   if (_that._is_byte == 1){

                        return   _that._watch.value.replace( /[^\x00-\xff]/g , "xx" ).length;   

                }   else {

                        return   _that._watch.value.length;    

                }

           },

           tip:   function (){

                   var   _that =   this ;

                   var   length = _that.get_len();

                   var   left = _that._limit - length;

                   if (left <= 0){

                     

                     _that._watch.value = _that._watch.value.substring(0, _that._limit   )

                }

                _that._tip.innerHTML  =   "还可以输入"   +left+ "个字符"   ;

                

           },

           init:   function (){

                   var   _that =   this ;

                _that._watch.onkeydown =   function (){

                     _that.tip();

                }

                   this ._watch.onkeyup =   function (){

                     _that.tip();

                }

           }

     }

     

      var   obj =   new   watching_limit({watch: "reason_note" ,tip:   "reason_note_tip" ,limit:10,is_byte:0});

     obj.init();