[Peaker]

If you care about the learning curve:

Take a look at https://github.com/sinelaw/inferno It aims to add a type system to a subset of Javascript. It's not quite ready yet, but it already shows quite some promise.

There's also Flow and Google's Closure/et-al, but they are more lenient and accept more valid and invalid programs.

[lmm]

If I just want types+js, Typescript is already pretty mature and widely supported (e.g. good integration with node, support for lots of popular libraries via DefinitelyTyped). What does this inferno offer over that?

[Peaker]

According to the author of Inferno, TypeScript copies all the "bugs" and error-prone behaviors of JS, and is also a superset, not a subset. Inferno being a subset lets you use all your JS tooling directly with Inferno code.