lazyProperty-example.js

/**
 * From AS2:
 * class A {
 *   var foo = 5;
 * }
 * 
 * class A extends B {
 *   var foo = 6;
 * }
 */
function A() {}
Object.defineProperties(A.prototype, {
  foo: lazyProperty('foo', function() { return 5; });
});

function B() {}
B.prototype = Object.create(A.prototype, {
  foo: lazyProperty('foo', function() { return 6; });
})

lazyProperty.js

function lazyProperty(propertyName, factory) {
  return {
    get: function() {
      return this[propertyName] = factory();
    },
    set: function(value) {
      Object.defineProperty(this, propertyName, {value: value, writable: true});
    }
  };
}