d3.js Key Function Default Identity Behavior Lacks Deterministic Constraint

How to make the default identity key deterministic and detect collisions

The default d3.js data‑join uses the array index as the key when no key function is supplied; this means that after reordering, filtering, or splicing the same datum may be bound to different nodes, causing unintended enter or exit transitions.

Confirmed facts: In d3‑selection (v4+), selection.data(data, key) binds by position unless a key function is provided; there is no built‑in warning for duplicate keys, and the join resolves silently, so later data items may consume existing nodes while earlier ones are removed.

1. Add a stable, unique identifier to each datum once (e.g., a UUID or a stored counter) and use it as the key function: selection.data(data, d => d.id).
2. Before binding, verify that the key function yields a unique value for every datum: map the keys into a Set and ensure the Set size equals the data length; if not, flag the duplicates.
3. Re‑bind after any data reorder or filter; compare selection.enter().size() and selection.exit().size() with the previous bind to confirm that nodes are updated rather than recreated.
4. Inspect the DOM nodes before and after the re‑bind; with a stable key the same elements should persist as updates, not as removed and re‑added nodes.

If you are using a d3 version older than v4, the API differs slightly, so please confirm the exact d3 major version you run.