var performanceS=function(){};
                performanceS.prototype.calculate=function(salary){
                    return salary*4;
                }
                var performanceA=function(){};
                performanceA.prototype.calculate=function(salary){
                    return salary*3;
                }
                var performanceB=function(){};
                performanceB.prototype.calculate=function(salary){
                    return salary*2;
                }

                var Bonus=function(){
                    this.salary=null;
                    this.strategy=null;
                };
                Bonus.prototype.setSalary=function(salary){
                    this.salary=salary;
                }
                Bonus.prototype.setStrategy=function(strategy){
                    this.strategy=strategy;
                }
                Bonus.prototype.getBonus=function(){
                    //这里的calculate是另外三个对象原型下的方法这为什么不报错，是怎么实现的
                    return this.strategy.calculate(this.salary);
                }

                var bonus=new Bonus();
                bonus.setSalary(10000);
                bonus.setStrategy(new performanceS());
                console.log(bonus.getBonus());//40000
                bonus.setStrategy(new performanceA());
                console.log(bonus.getBonus());//30000