In my experience, most of the benefits of Typescript come from type-checking across call boundaries, the point where type-related bugs are most likely to be introduced due to each side of the call often being in different locations and contexts. And you can't get those benefits without explicitly typing function parameters.
If you really don't want the compilation step; you can use JSDoc and get almost the best of both worlds (not everything in TS is supported by JSDoc but most essentials are)
