At the end of an interview, "Do you have any questions for us?" What should you ask?

There is no single right answer to this one, but there is a way to answer that wastes the chance: "No, thank you." Your question tells the interviewer what you care about, and their answer helps you decide whether you want to work there.

Prepare four or five questions in advance and pick two or three depending on the time left. Good questions are usually tied to a detail in the job ad or to something the interviewer just said:

• About the work: "In the first three months, what does a junior usually work on?", "How does the team review code, and how long does a pull request usually take to be merged?"
• About learning: "How often does a mentor meet a new hire?", "What does a junior who does well after the probation period look like, in your view?" The last one is especially useful, because the answer is exactly the standard you will be judged against.
• About the process: "After today, are there more rounds, and when could I expect feedback?"

What to avoid:

• Questions the job ad or the conversation has already clearly answered, because they show you weren't paying attention.
• Asking about salary or days off in a technical round with the team lead. Ask the recruiter, or in a later round.
• Generic questions copied from the internet when you don't really care about the answer.

Bonus points: ask a follow-up based on something the interviewer told you during the interview, for example "you mentioned earlier that the team is moving to Next.js, will juniors get to be part of that?" It shows that you listened and didn't just read out prepared lines.