JsonMapper built every JSON object it projects into script as an instance
of a private ObjectInstance subclass that existed only to be instantiable.
A host subclass can never carry the engine's shape-mode storage flag, so
each of those objects - all of ctx.data's leaf objects, and every JsonValue
var - sat permanently outside the own-property inline caches, and a script
reading the same property across a batch of content items re-resolved it
every time.
JsObject.CreateFromEntries builds the same object through the hidden class
machinery instead: repeated calls presenting the same key sequence, which
every content item of one schema does, share an interned hidden class, so
those reads stay monomorphic. The result is documented as indistinguishable
from the equivalent object literal - same own key order, same
configurable/enumerable/writable data properties - and anything the
representation cannot express (a digit-leading key, a very wide object)
falls back to the ordinary dictionary representation rather than to
different behaviour.
That fallback is silent, which is why the shaping is asserted rather than
assumed: Engine.Advanced.HasSharedShape is the supported predicate for it,
and JsonMapperTests pins that the projected object and its nested objects
answer true. Building them as a host subclass again would fail that test.
Three smaller fixes in the same file:
- the reverse direction allocated a string key per array element
(a.Get(i.ToString(...))); the indexed accessor reads the dense backing
directly and keeps the prototype walk for a modified array;
- JsNumber.Create reuses cached instances for small integers where
new JsNumber always allocated;
- JsString.Create, public since 4.15.3, interns the empty and single
character strings where new JsString always allocated.
Pinned by the projection tests added first - own key order, JSON.stringify,
for..in, mutation including delete and add, and the round trip back to
JsonValue - which pass before and after.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016uV6H9cTntzsoKiaJRBn4f
Marko Lahma
11f7c9ab98