javascript如何定义私有方法

var Person = function(){   

    var salary = 0.0;

    this.setSalary = function(value){

        salary = value;

    }

    this.getSalary = function(){

        return salary;

    }

}

var p = new Person();

p.setSalary(1000);

alert(p.getSalary());//返回1000

alert(p.salary);//返回undefined