Both Javascript and Python are perfect for this, IMO - both languages have syntax for type annotations which are ignored by the intepreter, but are respected by

Both Javascript and Python are perfect for this, IMO - both languages have syntax for type annotations which are ignored by the intepreter, but are respected by external typecheckers. Mypy for Python (https://mypy.readthedocs.io/en/stable/ https://mypy.readthedocs.io/en/stable/) and Flow for Javascript (https://flow.org/en/docs/ https://flow.org/en/docs/) are my personal favorites, and have all the features you describe. And importantly, both languages have very strong library support.