jQuery plugin creation and declaration for usage with $(). or $
Since your function - as far as I can tell - doesn't rely on a jQuery collection, I'd stick to always using "1 true way" of calling it.
I.e. I assume that

$("some &gt; selector").formArrayToObject(arr) === $.formArrayToObject(arr);


that is, the two ways of calling it give the same exact result.

If that's the case, just use $.yourFunction and only that. Having two invocations that should be semantically different but actually aren't is just confusing.

For instance, jQuery doesn't have both $.ajax() and $(...).ajax() simply because the latter doesn't really make sense; the $.ajax function isn't dependent on a certain context or selector, so why would you call it as if it was?

(besides, your second declaration in the code above is broken; it ignores arguments)