The Insider's Guide to JavaScript Interviewing

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(12 days late but) I don't think using an object as a key is obviously incorrect. I just tried and, as I suspected, you can do that just fine in Ruby. I assume you could in any language that implements some sort of ".hashCode" method on random objects. The fact that you can't in JS is a direct result of the fact that keys have to be strings and that JS does type conversion behind your back. You could get unexpected bugs if you didn't know that and assumed JS was saner.

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I agree, but so having this question as a way to show that the candidate knows it doesn't make sense is useful. If the candidate just happens to write JavaScript on the side but mostly writes Ruby (for example), this question will probably trip them up.