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Weird behavior with Javascript getter/setters
var Record = function(data) {
  this.fieldValues = {}
  this._data = data;
  var record = this;
  for(var key in data) {
    record.__defineGetter__(key, function() {
      return record._data[key];
    });
    record.__defineSetter__(key, function(val) {
      record.fieldValues[key] = val;
    });
  }
}

var myRecord = new Record({prop1: 'prop1test', prop2: 'prop2test'});

myRecord.prop1 = 'newvalue';

console.log(myRecord.fieldValues.prop1); // undefined
console.log(myRecord.fieldValues.prop2); // 'newvalue'

var Record = function(data) {
    var key;
    this.fieldValues = {}
    this._data = data;
    for(key in data) {
        //closure maintains state of "key" variable
        //without being overwritten each iteration
        (function (record, key) {
            record.__defineGetter__(key, function() {
                return record._data[key];
            });
            record.__defineSetter__(key, function(val) {
                record.fieldValues[key] = val;
            });
        }(this, key));
    }
}