Idea behind adding GADT to squishy-pants, it's not a nice as other languages, but this is the best we can do currently in a dynamic world.

squishy-pants-gadt.js

var a = _.List(_.Some(_.Right(1)), _.Some(_.Left('Error')));
var result = a.match({
  Some: { 
      Right: partial(
          function(a) {
              return a == 1; 
          },
          _.identity
      ),
      Right: _.constant(-1),
      _: _.constant(-1)
  },
  None: _.constant(-1);
});