Coercion · identity · order · handoff

Object.groupBy vs Map.groupBy key lab

The lab compares grouping semantics. It does not clone members, sanitize values, preserve object identity through JSON, or claim a universal performance winner.

Ready.

Shared-reference proof

Run a fixture, then mutate and reset the first grouped row.

Object.groupBy

Property-key groups and member row IDs
Order	Typed key	Members	Access	Serialization

Map.groupBy

Identity/SameValueZero groups and member row IDs
Order	Typed key	Members	Access	Serialization

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