I remember this topic came up before - so how do we make them visible? eslint, the JavaScrpt checker/formatter can be set up to run and fix/format code on save. The same could be done for adding types. To be safe, they could be added as comments such as in Flow. Or, the editor could show the type of expressions or functions in a side panel of the code, or as metadata above each expression or function (not part of the code, but like Xcode inline errors).