Skip to main content


Deno standard library
Go to Latest

Printf for Deno

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?

  • behaviour of %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 and 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 comma) (< Mnemonic: pull each element out of array)

  • how to deal with more newfangled JavaScript features (generic Iterables, Map and Set types, typed Arrays, …)

  • 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)