deepFreeze

obj1 = {
  internal: {}
};

Object.freeze(obj1);
obj1.internal.a = 'aValue';

obj1.internal.a // 'aValue'

// To make obj fully immutable, freeze each object in obj.
// To do so, we use this function.
function deepFreeze(obj) {

  // Retrieve the property names defined on obj
  var propNames = Object.getOwnPropertyNames(obj);

  // Freeze properties before freezing self
  propNames.forEach(function(name) {
    var prop = obj[name];

    // Freeze prop if it is an object
    if (typeof prop == 'object' && prop !== null)
      deepFreeze(prop);
  });

  // Freeze self (no-op if already frozen)
  return Object.freeze(obj);
}

obj2 = {
  internal: {}
};

deepFreeze(obj2);
obj2.internal.a = 'anotherValue';
obj2.internal.a; // undefined