This is very much a work-in-progress. I'm actively soliciting feedback. What immediately follows are points for discussion.
What useful features are available in other languages apart from Golang and C?
%v verb. In Golang, this is a shortcut verb to "print the
default format" of the argument. It is currently implemented to format using
toString in the default case and
inspect if the
%#v alternative format
flag is used in the format directive. Alternatively,
%V could be used to
distinguish the two.
inspect output is not defined, however. This may be problematic if using
this code on other platforms (and expecting interoperability). To my
knowledge, no suitable specification of object representation aside from JSON
toString exist. ( Aside: see
"Common object formats" in
the "Console Living Standard" which basically says "do whatever" )
%j verb. This is an extension particular to this implementation. Currently
not very sophisticated, it just runs
JSON.stringify on the argument.
Consider possible modifier flags, etc.
< verb. This is an extension that assumes the argument is an array and will
format each element according to the format (surrounded by  and separated by
< Mnemonic: pull each element out of array)
the implementation is fairly rough around the edges:
currently contains little in the way of checking for correctness. Conceivably, there will be a 'strict' form, e.g. that ensures only Number-ish arguments are passed to %f flags.
assembles output using string concatenation instead of utilizing buffers or other optimizations. It would be nice to have printf / sprintf / fprintf (etc) all in one.
float formatting is handled by toString() and to
toExponential along with a
mess of Regexp. Would be nice to use fancy match.
some flags that are potentially applicable ( POSIX long and unsigned modifiers are not likely useful) are missing, namely %q (print quoted), %U (unicode format)