immutable.js

    _prepare_environment: function _prepare_environment() {
        var env = this.options.env || {};

        delete this.options.env;

        this.env = _.bind(function() {
            return _.clone(env);
        });
    },

immutable2.js

    _prepare_environment: function _prepare_environment() {
        var env = this.options.env || {};

        delete this.options.env;

        deepFreeze(env);
        Object.defineProperty(this, 'env', {
                   value: env,
               writeable: false,
              enumerable: true,
            configurable: false
        });
    },

zz_deep_freeze.js

/**
 * Performs a deep (recursive) freeze on the object preventing
 * not only the object from being modified, but any decending
 * properties as well.
 *
 * Source (4/22/2014):
 * https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/freeze#Examples
 *
 * Browser Compatibility
 *
 * IE9+, Fx 4+, Chr 6+, Saf 5.1+
 *
 * @param  {Object} o
 */
function deepFreeze(o) {
  var prop, propKey;
  Object.freeze(o); // First freeze the object.
  for (propKey in o) {
    prop = o[propKey];
    if (!o.hasOwnProperty(propKey) || !(typeof prop === "object") || Object.isFrozen(prop)) {
      // If the object is on the prototype, not an object, or is already frozen, 
      // skip it. Note that this might leave an unfrozen reference somewhere in the
      // object if there is an already frozen object containing an unfrozen object.
      continue;
    }

    deepFreeze(prop); // Recursively call deepFreeze.
  }
}

/**
 * Defines a property on an object and makes it immutable such that
 * it, or its descending objets cannot be changed in any way.
 * The object can still be deleted
 *
 * @param  {Object} obj   Object on which to add the property
 * @param  {String} name  Name of property
 * @param  {Object} value Value to set the property to
 */
function defineImmutableProperty(obj, name, value) {
    deepFreeze(env);
    Object.defineProperty(obj, name, {
               value: value,
           writeable: false,
          enumerable: true,
        configurable: true
    });
}