Shared array between class instances in node.js

Question

I have a a weird problem in node.js:

person.js

var Person;

Person = (function() {
  Person.prototype.name = "";
  Person.prototype.friends = [];

  function Person(name) {
    if (name) {
      this.name = name;
    }
  }

  Person.prototype.sayHello = function() {
    return console.log("Hello, my name is " + this.name + " and I have " + this.friends.length + " friends");
  };

  Person.prototype.addFriend = function(name) {
    this.friends.push(name);
  };

  return Person;

})();

module.exports = Person; 

factory.js

var Person = require('./Person.js');

module.exports = function(name) {
  return new Person(name);
};

index.js

factory = require('./factory');

tyrion = factory("Tyrion");
tyrion.addFriend("Bronn");
tyrion.sayHello();
daenerys = factory("Daenerys");
daenerys.addFriend("Illyrio");
daenerys.addFriend("Daario");
daenerys.addFriend("Barristan");
daenerys.sayHello();
tyrion.sayHello();

Actual output

Hello, my name is Tyrion and I have 1 friends
Hello, my name is Daenerys and I have 4 friends
Hello, my name is Tyrion and I have 4 friends

Expected output

Hello, my name is Tyrion and I have 1 friends
Hello, my name is Daenerys and I have 3 friends
Hello, my name is Tyrion and I have 1 friends

How come that adding element to one instance does add it for both? It looks like the friend array is "shared" between instances. How to prevent such?

Demo here

Answer 1

Remove lines

Person.prototype.name = "";
Person.prototype.friends = [];

add them to constructor instead:

this.name = name;
this.friends = [];

At the moment all prototypes share same object friends.