It's a weird choice that this library makes some JS APIs more strict, and others less strict. Specifically, the changes to JSON/Array.isArray make sense, the value is unknown, and coercing it to any encourages leaking that type further, or runtime errors. However, the changes to includes/Set basically amount to "I said that this array contained this enum, but I changed my mind when I wrote this if statement". This actually allows more bugs to slip through, rather than fewer.