gistfile1.js

var sys = require('sys'),
    buffer = '{"keys":[1,2,3]}';

sys.p(JSON.parse(buffer));
sys.puts("Before keys extension worked fine.");

function extend (obj, props) {
  Object.getOwnPropertyNames(props).forEach(function(prop){
    var descriptor = Object.getOwnPropertyDescriptor(props, prop)
    descriptor.enumerable = false
    Object.defineProperty(obj, prop, descriptor)
  })
}

extend(Object.prototype, {
  get keys() {
    return Object.keys(this)
  },
  
  set keys(val) {
    // Original implimentation which breaks
    // this.__defineGetter__('keys', function() {
    //   return val
    // });
  
    var self = this,
        setter = function(val) {
          self.__defineGetter__('keys', function() {
            return val;
          });
          self.__defineSetter__('keys', setter); // Breaks w/o this line
        };

    setter(val); 
  }
});

sys.p(JSON.parse(buffer));
sys.p(JSON.parse(buffer).keys);
sys.puts("If you see this, it worked. If not, it's busted.");