使用prototype属性来减少重复的代码

function Bird(name){
    this.name = name;
}

var bird1 = new Bird("bird1");
var bird2 = new Bird("bird2");
var bird3 = new Bird("bird3");
var bird4 = new Bird("bird4");

bird1.numLegs = 2;
...
...

Bird.prototype.numLegs = 2;

function Bird(name){
    this.name = name;
    this.numLegs = 2;	//直接在构造函数中添加了这个属性
}

console.log(bird1.numLegs);	//输出2
console.log(bird2.numLegs);	//输出2
console.log(bird3.numLegs);	//输出2
console.log(bird4.numLegs);	//输出2
...