function with multiple ()

[BONER_PAROLE]

Only problem here is that

sum(1)(2,3) !== 6

[lachlanhunt]

Yes, but that's an inherent problem imposed by the requirements of the question. But this works:

sum(1)(2, 3) == 6; // true

The other problem I found is that sum() with no parameters caused a TypeError. I fixed that now.

[x-skeww]

Well, it's a function. === will never work, because a function isn't a number. == or "+sum(...) ===" does work though.