How to specify the correct type?

Question

typescript

I'm trying to make a function that would accept as a key - only the keys of an object of type string. It turned out to be done, but inside the function I cannot use key as an index. As I understand it, before using, you need to make sure that it's not never? Tell me where to dig.
Code:

Sori that screen, there is no other now

Answer 1

https://www.typescriptlang.org/play?#code/PTAEHUFM...
Here is the solution, how will I bring it to normal at the computer, it's very inconvenient to type from the phone)