闭包

var obj = (function(){

     var i = 0;

    return{

      back1:function(){

        i++;

       alert(i)  

     },  
     back2:function(j){
               //修改obj函数里i的值
        i = j;
     }

  }

})()

obj.back1();
//外部修改i的值
obj.back2(4);
obj.back1();