Pergunta de entrevista da empresa BandPage

I was asked a JavaScript coding question that I had never been asked before. It was difficult, and took a good 30 minutes to solve. Out of respect for BandPage's interviewing process (because they probably still use the question) I won't reveal the exact details. It was very relevant to the position and a good judge of my expertise in JavaScript coding skills. The type of question was: implement this highly abstract and highly useful utility that you've probably been using in your work from a 3rd party library. It was really interesting to work out how that type of code functions at a low level. I learned something new by solving the problem.