I like you can type check stuff like this: type State = { id: "initial" } | { id: "running", speed: number } const f = (s: State) => { switch (s.id

I like you can type check stuff like this: type State = { id: "initial" } | { id: "running", speed: number } const f = (s: State) => { switch (s.id) { case "initial": // s.id available break; case "running": // s.id and s.speed available break; } } The problem with Typescript shows up when people with background in Java or C# starts taking oo design patterns into js-land where they don't really belong (patterns like dependency injection, factories, singletons etc.).