Index
if spidermonkey was built with js_c_strings_are_utf8 defined or js_setcstringsareutf8 was called, the string is converted to utf-8.

JSAPI User Guide
these functions convert their char * arguments to 16-bit strings by zero-extending each 8-bit char to 16 bits—unless js_c_strings_are_utf8 is defined or js_setcstringsareutf8 has been called, in which case each char * string is interpreted as utf-8 unicode text.

JS_CStringsAreUTF8
there are two ways to enable this: at compile time, by building spidermonkey with js_c_strings_are_utf8 defined; or at run time, by calling js_setcstringsareutf8 before the first call to js_newruntime.

JS_EncodeCharacters
if spidermonkey was built with js_c_strings_are_utf8 defined or js_setcstringsareutf8 was called, the string is converted to utf-8.