I'm pretty surprised to hear this from someone who taught a programming course. It actually makes a ton of sense when you evaluate what is happening: it's the negative unary value and then the negative unary result of that value - there's no possible way it could be anything other than true or false. It's more of a side affect of the unary operator ! than it is a language feature, which I believe is why it sees such ubiquitous implementation.

> I would leave a note to a student for using this to be honest - too unclear, and (as this thread denotes) not well known enough to put in production worthy code.

You've done a total disservice to your students that understood negative unary operations then. Given how many upvotes my initial post on this thread has (currently 38) I don't think the thread denotes what you think it does at all.

You're kidding, right? "!!foo" is a complete no-op in Java. It is never needed and never correct.

You're basically wrong on every single point. It's a syntactic hack in JS to get a guaranteed boolean coersion, but for anything else it's an absolutely pointless abomination. (Maybe PHP has a similar thing, but I haven't programmed in PHP, so I wouldn't know.)