令人耳目一新的鸭子

	var duck = {
		duckSinging: function(){
			console.log('嘎嘎嘎');
		}
	};
	var children = {
		duckSinging:function(){
			console.log('嘎嘎嘎');
		}
	};
	var choir = []; //合唱团
	var joinChoir = function(animal){
		if(animal && animal.duckSinging === 'function'){
			choir.push(animal);
			console.log('恭喜加入合唱团');
			console.log('合唱团已有成员数量：' + choir.length);
		}
	};
	joinChoir(duck);
	joinChoir(chicken);