When you convert sets to lists, you run the risk of adding values to the list out of order. Sets have no defined order, because they're not indexed collections. Immutable.js has an OrderedSet collection type that acts like an OrderedMap. It preserves the insertion order of its values. Other than that, it is just like a regular set.
Internally, Set uses Map to store its values as map keys. OrderedSet uses the OrderedMap keys.