[eyelidlessness]

> There is no hard guarantee at runtime

You can get very close if you place fine-grained, well tested, type guards at IO boundaries, wrap poorly typed or overly dynamic dependencies, and have good discipline about internal boundaries. It’s a lot of work up front in a greenfield project, but it really pays off.

> The types are not used to produce faster code at runtime

Not directly, but in my experience good up front interface design tends to produce either immediately optimal JS (eg it tends to be monomorphic) or makes optimization much easier (as it makes any refactoring easier).

Edit:

> I like type systems as you can design with types and check things tie together before even running the code. This gives you very fast iteration cycles compared to CMD-R refreshing the browser/code.

I’d be remiss not to emphasize this! And not just skipping the reload cycle, it lets you skip a whole lot of test runs too. Type error in editor? Yep, those tests are gonna fail too.