Shared mutable state among "instances of a class" in javascript.

class-variable.js

// In this gist, I explain how to achieve mutable shared state
// between "instances of a class", i.e. objects with a shared
// prototype.  This mimics how "class variables" are used 
// in other languages with classical inheritance. 

// Not something you want to do all the time, obviously, but it could
// be handy in some situations.

function Cat() {
  // increment the class variable each time a constructor is used.
  this.constructor.prototype.catsSoFar += 1;
}
Cat.prototype.constructor = Cat;
Cat.prototype.catsSoFar = 0;

// All instances will see the change in this variable.
garfield = new Cat();
garfield.catsSoFar // 0
tom = new Cat();
felix = new Cat();
garfield.catsSoFar // 3