I feel like I can somewhat relate, I've been adding flowtype [0] annotations to my JS code, which is transpiled with babel. However, since I'm using unsupported ES2015 features and experimental ES2016+ features, it fails to run over my code (which is completely understandable).

The workaround I've found is to use a plugin [1], which converts the type annotations into runtime checks. It's not perfect, but I've found it helps catch some bugs quickly. It's also really helpful for documentation purposes: you know that this function param takes a number param, if you're not passing in a number you've made a mistake. Also, having the inline type is nicer than having it in a jsdoc-style comment above.

I think there's diminishing returns as you add more annotations to your code and you make them stricter, but having basic sanity checks has been helpful in catching stupid mistakes quickly.

[0] http://flowtype.org/ [1] https://github.com/codemix/babel-plugin-typecheck