JS_MakeStringImmutable
description a string's characters can never be changed, but spidermonkey uses two string optimization techniques behind the scenes: a growable string (see js_newgrowablestring ) has a buffer that the javascript engine can reallocate so that concatenating it with another string is much faster.

JSAPI reference
monkey 1.8.5 js_putescapedstring added in spidermonkey 1.8.5 js_putescapedflatstring added in spidermonkey 1.8.5 js_forget_string_flatness added in spidermonkey 1.8.5 js_stringisflat added in spidermonkey 38 js_assert_string_is_flat added in spidermonkey 1.8.5 js_flattenstring added in spidermonkey 1.8.5 typedef jschar obsolete since jsapi 35 js_newstring obsolete since jsapi 1.8.5 js_newgrowablestring obsolete since jsapi 27 js_getstringbytes obsolete since jsapi 1.8.5 js_getstringbytesz obsolete since jsapi 1.8.5 js_getstringchars obsolete since jsapi 1.8.5 js_getstringcharsz obsolete since jsapi 33 js_getstringcharsandlength added in jsapi 1.8.5 obsolete since jsapi 33 js_getstringcharsz added in spidermonkey 1.8.5 obsolete since jsapi 33 js_getstringcharszandlength added in spidermon...

SpiderMonkey 1.8.5
jsautorequest jsautosuspendrequest jsautocheckrequest jsautoentercompartment js::anchor<> js::call obsolete apis js_clearnewbornroots js_enterlocalrootscope js_leavelocalrootscope js_leavelocalrootscopewithresult js_forgetlocalroot js_newgrowablestring deleted apis js_addnamedroot – use js_add*root js_addnamedrootrt – use js_add*root js_addroot – use js_add*root js_clearnewbornroots – no longer needed js_clearoperationcallback js_clearregexproots js_decompilescript js_destroyscript js_enterlocalrootscope js_executescriptpart js_forgetlocalroot js_getfunctionname js_getoperationlimit js_getscriptobject js_getstringbytes ...

SpiderMonkey 1.8.7
jsautorequest jsautosuspendrequest jsautocheckrequest jsautoentercompartment js::anchor<> js::call obsolete apis js_clearnewbornroots js_enterlocalrootscope js_leavelocalrootscope js_leavelocalrootscopewithresult js_forgetlocalroot js_newgrowablestring deleted apis js_getscopechain use js_getglobalforscopechain api changes operation callback js_setoperationcallback was introduced in js 1.8.0, replacing the branch callback, in anticipation of the addition of the tracing jit (tracemonkey).

SpiderMonkey 31
obsolete apis js_convertarguments "j" type deleted apis js_newgrowablestring (can be replaced with js_newucstring) js_isconstructing (can be replaced with callargs::isconstructing or callreceiver::isconstructing) js_valuetoboolean (replaced by js::toboolean) js_valuetonumber (can be replaced with js::tonumber) js_valuetoint64 (replaced by js::toint64) js_valuetouint64 (replaced by js::touint64) js_valuetoecmauint32 (replaced by js::touint32) js_valuetoecmaint32 (re...