Make an object propertiable.

makepropertiable.js

function makePropertiable(obj) {

  var getProps = function () {
      this.__props = this.__props || {};
      return this.__props;
    },
    emptyGet = function (currentValue) {
      return currentValue;
    },
    emptySet = function (newValue) {
      return newValue;
    },
    emptyOnset = function (newValue) {
      // do nothing
    },
    emptyValidate = function (newValue) {
      return true;
    },
    validateKeyExistence = function (key) {
      var props = getProps.call(this);
      if ( ! (key in props)) {
        throw new Error('Property "' + key + '" does not exist.');
      }
    };

  obj.defineProp = function (key, attr) {
    var prop = {
      value: attr.value,
      get: attr.get || emptyGet,
      set: attr.set || emptySet,
      validate: attr.validate || emptyValidate,
      onset: attr.onset || emptyOnset
    };

    getProps.call(this)[key] = prop;
  };

  obj.set = function (key, value) {
    validateKeyExistence.call(this, key);

    var prop = getProps.call(this)[key];

    if (prop.validate()) {
      prop.value = prop.set.call(this, value);
      prop.onset.call(this, prop.value);

    } else {
      throw new Error('The value "' + value + '" of "' + key + '" is invalid.');
    }
  };

  obj.get = function (key) {
    validateKeyExistence.call(this, key);

    var prop = getProps.call(this)[key];

    return prop.get.call(this, prop.value);
  };

}