Subj : implementing sprintf in Javascript
To   : netscape.public.mozilla.jseng
From : Jason
Date : Mon Feb 21 2005 01:57 pm

hmm, in searching the archives this (creating a Javascript-accessible
sprintf) has come up several times (there was one posting 6/11/2001
labeled "JS-eng Tricks"), but I couldn't find anything that looked like
a good bulletproof solution.

has anyone who would be willing to share their implementation done this
in a robust manner?

The thread I referenced above constructed a va_list, used vsprintf, &
only supported INT and STRING types (leaving out double/float). Seems
like the caveat here is that both the C *printf functions and the
functions in jsprf.c assume correct matching of args & format
parameters & don't error-check.

(are JS_vsmprintf & the other functions in jsprf.c formal JSAPI
functions or is it one of the helper functions in SpiderMonkey subject
to change? They're not listed in the API docs
http://www.mozilla.org/js/spidermonkey/apidoc/sparse-frameset.html)

At least two issues here:
(1) resolving valid conversion of datatypes e.g. sprintf("%d %f",3,3)
(2) dealing with boneheaded mistakes w/o crashing e.g.
sprintf("%s",42);

My guess is that for either of these, you'd have to explicitly look
over the format string and cast JSVAL args appropriately (or throw an
error), then either hand them to one of the sprintf functions or
manually implement the printf itself (bleah.)

.