Kinda nitpicky. Sure ADTs are a pattern in Typescript rather than a first-class entity, but they're an extremely well-supported pattern, exhaustiveness checking and all.

    type ADT = 
        | { case : 'a', a : Number }
        | { case : 'b', b : string }

    function f (c : ADT) {
        switch(c.case) {
            case 'a' : return c.a
            case 'b' : return Number.parseInt(c.b)

            // case 'c' : return 0 // compile error

            // case 'b' : return c.a // also compile error
        }
    }