Function uses_side_table 

pub fn uses_side_table(v: &Value) -> bool

Object(x): box/pass-through — for our model, non-object args just return a fresh object; objects pass through. Whether v’s own properties live in the fn-prop SIDE TABLE rather than in a property map. A Map/Set/Promise/RegExp/generator/symbol/bigint is an ordinary object that also has internal slots, so it can carry own properties like anything else — but its heap variant holds only those slots, so a write had nowhere to go and vanished: m.x = 5 left m.x undefined.