> However, even if the db object is not reassignable, we can still modify its internals, as long they are declared as let and var. That’s why we will mock single functions and not the whole object.

this doesn't work for the awful practice encouraged by eslint (at least when i last tried it) that consists of ignoring that there exists a keyword specifically for declaring functions, and abusing constants with arrow functions instead

sure, it prevents you from reassigning the function variable and using `this` but aren't those things what you use a linter for?