I think this question is very revealing when paired with another related question. The rating is unimportant and not to be used as a requirement, anyone can lie

I think this question is very revealing when paired with another related question. The rating is unimportant and not to be used as a requirement, anyone can lie and say 10. I feel that it shows insight into how strong a candidate really is and how much they are bluffing, for example “rate yourself in JavaScript” and then asking a JS question. If their answer lines up with their self-assessment (somewhere in the ballpark) then it seems more likely their other claims are true. However, I would never ask this question and rate a candidate on whatever number they decide to come up with. Lots of times the most skilled engineers rate themselves lower than you would expect. So yeah, the number is meaningless, but I think the question can be revealing in an interview setting.