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Type Annotations
Static type checking can be achieved in CoffeeScript by using Flow’s Comment Types syntax:
codeFor('type_annotations')
CoffeeScript does not do any type checking itself; the JavaScript output you see above needs to get passed to Flow for it to validate your code. We expect most people will use a build tool for this, but here’s how to do it the simplest way possible using the CoffeeScript and Flow command-line tools, assuming you’ve already installed Flow and the latest CoffeeScript in your project folder:
coffee --bare --no-header --compile app.coffee && npm run flow
--bare and --no-header are important because Flow requires the first line of the file to be the comment // @flow. If you configure your build chain to compile CoffeeScript and pass the result to Flow in-memory, you can get better performance than this example; and a proper build tool should be able to watch your CoffeeScript files and recompile and type-check them for you on save.
If you know of another way to achieve static type checking with CoffeeScript, please create an issue and let us know.