Proxy Invariants
Proxies can’t lie about everything. The JavaScript engine enforces invariants — consistency rules between the proxy’s behaviour and the target’s state.
If a property is non-configurable and non-writable on the target, the get trap must return the actual value — not a fabricated one. If Object:is-extensible target returns false, the proxy’s is-extensible trap must also return false.
These invariants prevent proxies from creating impossible objects. A proxy can intercept and customize, but it can’t contradict the target’s non-configurable properties. The target’s constraints are the proxy’s constraints.
The reader doesn’t need to memorize every invariant. The key insight: proxies customize behaviour, they don’t override reality.