I suspect the problem you have is that you are overriding Sammy's scope by using the fat-arrow syntax (which preserves your lexical scope).
var app: Sammy.Application = Sammy();
app.get('#:foobar', function () {
//doStuff
var baz = this.params.foobar;
});
By using "function" instead of "() =>" you avoid scope preservation and allow Sammy to work as usual.