It doesn't HAVE to, but usually libraries designed with a JS mindset don't lend themselves to type safety very well (JS coders like their magic strings and very dynamic types). Somehow React still managed pretty decently in that area whereas vueJS is a catastrophe.
I'd be interested to know how many DefinitelyTyped contributors agree with the "usually" part. My impression is that problems specifying types for existing libraries are the exception rather than the rule.
Many non-trivial libs are typed improperly, even when they can be done right. There's massive abuse of "any" type the moment anything gets semi-challenging. Also, a non-trivial amount of type definitions don't work in strict mode, which make things worse.
Anything involving complicated higher order functions (like lodash/ramda's curry), or having a single method that can be used in various ways that don't fit well with TS' overloading mechanisms are often done wrong or poorly. Flow is a bit better at this, but the techniques it provides are not always well known, so the type definitions aren't that much better.
