[dangoor]

    const a = icepick.freeze({b: "foo"});

Granted, that's not as elegant as built-in support but that is literally what I'm doing at the moment when I want immutability.

[Touche]

So if you go to extraordinary lengths you can use JavaScript functionally but it's not very functional by default aside from the first-class functions. Personally I just embrace it's imperative/functional blended nature and go with it.