nyan.js

function Person(firstName, lastName){
  this.firstName = firstName;
  this.lastName = lastName;
}

var p = new Person("shinpei", "maruyama");

function nyan(p){
  p = new Person("tarou", "yamada");
}
nyan(p);
console.log(p.firstName); // shinpei
console.log(p.lastName);  // maruyama

function wan(p){
  p.firstName = "tarou";
  p.lastName  = "yamada";
}
wan(p);
console.log(p.firstName); // tarou
console.log(p.lastName);  // yamada