For me, it's first class function support. Of course, there are other "real functional languages" like Scala, Haskel or whatever, but they very uncommon.
JS is the only commonly (Clojure is very uncommon) used language with real first class functions and not some silly hack like Ruby/Python lamdas, procs, or blocks.
To be fair, it seems that Java 8 has first class functions. I can't say much about it since I didn't get a chance to play with it yet.
To be fair, it seems that Java 8 has first class functions. I can't say much about it since I didn't get a chance to play with it yet.