There are a couple of reasons why the 'function that swaps numbers' question is, in my opinion, a bad interview question.
1. it's a very simple thing to do, so why bother with a function? It's like asking someone to write a function that takes an integer value and returns an array of that length.
2. if you really do need a function that swaps numbers, then you probably also have some special edge cases that you will have to deal with that the candidate likely won't know about
3. each solution is language dependent
Maybe those aspects are why you consider it a good question, but when I have been asked those types of questions in the past I felt that the interviewer was missing an opportunity.
1. it's a very simple thing to do, so why bother with a function? It's like asking someone to write a function that takes an integer value and returns an array of that length. 2. if you really do need a function that swaps numbers, then you probably also have some special edge cases that you will have to deal with that the candidate likely won't know about 3. each solution is language dependent
Maybe those aspects are why you consider it a good question, but when I have been asked those types of questions in the past I felt that the interviewer was missing an opportunity.