Can I freeze a Set or Map in JavaScript?

Freezing an object is rather simple, using Object.freeze(). However, when it comes to freezing a Set or Map, things get a bit more complicated. The Set and Map objects have their own methods for adding, deleting and clearing elements, which can be problematic when trying to freeze them.

Luckily, as they're objects after all, you can easily override these methods to prevent them from being used.

Freeze a Set object

In order to freeze a Set object, you can simply set the Set.prototype.add(), Set.prototype.delete() and Set.prototype.clear() methods to undefined. This will effectively prevent them from being used, practically freezing the object. In addition to that, you can use Object.freeze() to freeze the Set object itself.

const frozenSet = iterable => {
  const s = new Set(iterable);
  s.add = undefined;
  s.delete = undefined;
  s.clear = undefined;
  return Object.freeze(s);
};

frozenSet([1, 2, 3, 1, 2]);
/* Set {
  1, 2, 3,
  add: undefined, delete: undefined, clear: undefined
} */

Freeze a Map object

Freezing a Map object is very similar to freezing a Set object. You can set the Map.prototype.set(), Map.prototype.delete() and Map.prototype.clear() methods to undefined and then use Object.freeze() to freeze the Map object itself.

const frozenMap = iterable => {
  const m = new Map(iterable);
  m.set = undefined;
  m.delete = undefined;
  m.clear = undefined;
  return Object.freeze(m);
};

frozenMap([['a', 1], ['b', 2]]);
/* Map {
  'a' => 1, 'b' => 2,
  set: undefined, delete: undefined, clear: undefined
} */