In Mascara (my own project) you can start out with dynamically typed JavaScript, but then gradually add type annotation as appropriate to provide stronger verif

In Mascara (my own project) you can start out with dynamically typed JavaScript, but then gradually add type annotation as appropriate to provide stronger verification. It can be done using structural types without changing the runtime semantics, and further by rewriting to use classes and nominal typing. I think this provides a useful upgrade path, because type verification is a hassle with small projects, but becomes increasingly valuable (IMHO) as the program grows. The problem with current languages is that you have to decide upfront if you want a language optimized for quick development or strong verification. But often in the real world, programs start out as quick prototypes, and then grow into large applications.