JavaScript class implementation (with example PersonClass declaration)

class.js

var Class = function(base){
  if (typeof base.construct !== "function")
    throw new TypeError("class definition has to define 'construct' function");
  return function() {
    // copy all methods
    for (var prop in base) {
      if (base.hasOwnProperty(prop) && base[prop] !== base.construct) {
        this[prop] = base[prop];
      }
    }
    // call the constructor
    var args = Array.prototype.slice.call(arguments);
    base.construct.apply(this, args);
  }
};

var personDefinition = {
  construct: function construct(firstname, lastname) {
    this.firstname = firstname || this.firstname;
    this.lastname = lastname || this.lastname;
  },
  firstname: 'John',
  lastname: 'Doe',
  getFullName: function() {
    return this.firstname + ' ' + this.lastname;
  }
};

var Person = Class(personDefinition);

var person1 = new Person("John", "Lennon");
var person2 = new Person("Paul", "McCartney");
var person3 = new Person("Saint");
var person4 = new Person();

person1 instanceof Person; // true
person1.getFullName(); // 'John Lennon'

person2 instanceof Person; // true
person2.getFullName(); // 'Paul McCartney'

person3 instanceof Person; // true
person3.getFullName(); // 'Saint Doe'

person4 instanceof Person; // true
person4.getFullName(); // 'John Doe'