JS_GetFunctionFlags
jsfun_heavyweight the function uses language features such as closures, with, or eval that require extra work on the part of the interpreter.

JSAPI reference
macros js_default_xml_namespace_id obsolete since jsapi 21 jsfun_bound_method obsolete since javascript 1.8.5 jsfun_getter obsolete since javascript 1.8.5 jsfun_setter obsolete since javascript 1.8.5 jsfun_global_parent obsolete since javascript 1.8.5 jsfun_heavyweight obsolete since jsapi 19 jsfun_lambda obsolete since jsapi 19 c++ features class jsautorequest class jsautolocalrootscope obsolete since javascript 1.8.5 class js::perfmeasurement (in jsperf.h) tracing and debugging js_setfunctioncallback added in spidermonkey 1.8.5 obsolete since jsapi 37 js_getfunctioncallback added in spidermonkey 1.8.5 obsolete since jsapi 37 ...