JavaScript はコンストラクター モードとプロトタイプ モードを組み合わせます。

function testPrototype2(){
  function Person3(name, age, job){
    this.name=name;
    this.age=age;
    this.job=job;
    this.friends =["shelb", "court"];
  }
  Person3.prototype = {
    constructor:Person3,
    sayName:function(){
      alert(this.name);
    }
  }
  var person1 = new Person3("jack",10,"it");
  var person2 = new Person3("karry",1,"woker");
  person1.friends.push("tom");
  console.info(person1.friends);
  console.info(person2.friends);
  console.info(person1.friends==person2.friends);
  console.info(person1.sayName == person2.sayName);
}