function clock(){

     this.min    = 3; //倒数的分钟

     this.speed  = 1000; //setTimeout时间

     var timeId  = ""; //保存setTimeout对象

     this.time   = this.min * 60;//3分钟等于180s

     this.move   = function(){

         var that    = this;//保存当前对象this

         this.time   = parseInt(that.time);

         var minute  = 0; //分

         var seconds = 0; //秒

         if(this.time > 60){

             minute  = parseInt(that.time / 60);

             seconds = parseInt(that.time % 60);

         }else{

             minute  = 0;

             seconds = parseInt(that.time % 60)

         }

         var txt     = ((minute < 10) ? "0" + minute : minute) + " : " + ((seconds < 10) ? "0" + seconds : seconds);

         timeId      = setTimeout(function(){

             that.move();

         },that.speed);

         console.log(that.time);

         that.time--;

         console.log(txt);

         if(that.time <= 0){

             clearTimeout(timeId);

         };

     },

     this.stop = function(){

        clearTimeout(timeId);

     }

 }

 var eva3_clock = new clock();

     eva3_clock.move();