A somewhat simple way to have abstract classes in JavaScript. There's probably a better way to do this.

class_abstract.js

// Define an abstract class
var Abstract = function() {};
Abstract.prototype.f1 = function() {
  throw "Not implemented";
}
Abstract.prototype.f2 = Abstract.prototype.f3 = Abstract.prototype.f1;

// Implement our abstract class
var Class = Abstract;
Class.prototype.f1 = function() {
  return 1;
}

Class.prototype.f2 = function() {
  return 2;
}

// Use it
var test = new Class();

// Debug messages and stuff
console.log(test instanceof Class);
console.log("Result of f1(): " + test.f1());
console.log("Result of f2(): " + test.f2());
test.f3();