Are you saying that JS implementations somehow know whether the array is used to store immutable values, and will switch implementations to allow for sharing?
In JS, primitives are immutable -- number, functions (but not the fn object), regex, strings, and booleans. All other types are not immutable -- Object, Array (actually an Object), Map, Set, TypedArray, etc. Even if a linked list were added, it would not be immutable.
That said, JS has Object.freeze() which can make an object immutable. You could do this recursively to make a guaranteed immutable object. Immutable libraries in JS tend to use other methods for performance reasons though IIRC.
