I think it's important not to get _too_ caught up on terminology, though. Someone who is already familiar with say C, C++, Java, and Perl, HTML, and CSS, but has only done some basic javascript (just to choose a random grab-bag) might look at you with a blank stare if you ask them about a closure. However, they may well be familiar with the concept, as in Java an anonymous class has many of the same properties. And I would argue that such a person would probably be able to pick up javascript and become effective in it in a matter of days, provided that they already have an extensive development background in other languages.
If you truly have only a day or two to get somebody up to speed, then sure, you need somebody who is already intimately familiar with every technology that you use ... but if it's a gigantic project, you're probably going to have a bigger learning curve than a couple of days anyway.
However, if you're looking for the absolute best person you can get for a job, you shouldn't write them off because they haven't had experience with a specific technology. More important is that they have a solid understanding of algorithms, data structures, and various design architectures, have exposure to a number languages, and are capable of rapidly assimilating new concepts. I'll take the lightening fast learner over somebody who knows a few specific techs _really_ well any time, because the lightening fast learner will _also_ learn the specific quirks of my project quickly, whereas somebody who has a difficult time with new concepts but happens to know the right techs might struggle for a long time just to come up to speed on a large project that somebody else created.