JS_GetReservedSlot
syntax // added in spidermonkey 42 js::value js_getreservedslot(jsobject *obj, uint32_t index); void js_setreservedslot(jsobject *obj, uint32_t index, js::value v); // obsolete since spidermonkey 42 jsval js_getreservedslot(jsobject *obj, uint32_t index); void js_setreservedslot(jsobject *obj, uint32_t index, jsval v); name type description obj jsobject * an object that has reserved slots.
... v js::value (in js_setreservedslot) the value to store.
... see also mxr id search for js_getreservedslot mxr id search for js_setreservedslot bug 1184564 -- changed jsval to js::value ...

JSAPI User Guide
to protect the compiled script from garbage collection, the application creates a compiled script object by calling js_newscriptobject and makes that object gc-reachable using js_setproperty, js_setreservedslot, js_add*root, or some other function.

JSClass.flags
they can be accessed using js_getreservedslot and js_setreservedslot.

JSObjectOps.getRequiredSlot
note: the slot parameter is a zero-based index into obj slots, unlike the index parameter to the js_getreservedslot and js_setreservedslot api entry points, which is a zero-based index into the jsclass_reserved_slots(clasp) reserved slots that come after the initial well-known slots: proto, parent, class, and optionally, the private data slot.

JSAPI reference
struct jsclass jsclass.flags jsclass.call jsclass.construct struct jsfunctionspec struct jspropertyspec js_initclass js_linkconstructorandprototype added in spidermonkey 17 js::propertyspecnameissymbol added in spidermonkey 38 js::propertyspecnameequalsid added in spidermonkey 38 js::propertyspecnametopermanentid added in spidermonkey 38 js_getreservedslot js_setreservedslot struct jsextendedclass obsolete since javascript 1.8.5 struct jsobjectops obsolete since javascript 1.8.5 struct jsxmlobjectops obsolete since javascript 1.8.5 struct jsproperty obsolete since jsapi 16 adding native properties and methods to classes: jsnative struct jsfunctionspec js_fs added in spidermonkey 1.8 js_fn added in spidermonkey 1.8 js_sym_fn added in spidermonkey 38 j...