Types at the edge of your system

Coding with static types feels safe, the compiler has your back.

But today, you got a critical issue in prod: payments are failing! A quick check of the logs left you puzzled: TypeError: discount.toFixed is not a function. Wait... Isn't discount supposed to be a number here? How did that happen? Why didn't the compiler catch it before?! Worry no more.

I'll show you where your blind spots are, and a simple way for you to fix them.