Does core-js Map polyfill maintain SameValueZero consistency for NaN keys?

Answer

Yes, the core-js Map polyfill (version 3 and later) maintains SameValueZero consistency for NaN keys: multiple set calls with NaN as the key update a single entry rather than creating duplicates.

Confirmed facts

• The polyfill stores entries in an internal array and compares keys using a sameValueZero helper.
• That helper returns true for two values when both are NaN (x !== x && y !== y) or when Object.is(x, y) is true, which matches the ECMAScript SameValueZero specification.
• When Map.prototype.set is invoked with a NaN key, the polyfill scans the array with sameValueZero; if an existing NaN entry is found, it updates that entry’s value instead of pushing a new one.
• Consequently, after map.set(NaN, 'first'); map.set(NaN, 'second'); the map’s size is 1 and map.get(NaN) returns the last value set ('second').
• Because the comparison logic is pure JavaScript, the behavior is consistent across all target environments that load the polyfill.

Verification steps

1. Load core-js (e.g., via a CDN script tag for version 3.x).
2. Create a map: const map = new core.Map();
3. Execute:

map.set(NaN, 'first');
map.set(NaN, 'second');

4. Check:
• map.size should be 1.
• map.get(NaN) should return 'second'.
5. Optionally repeat with other values (0, -0, undefined) to confirm that only NaN follows the SameValueZero rule.

Missing diagnostic detail

To give a definitive recommendation, please confirm which version of core-js you are using. If you are on a pre‑v3 build, the internal implementation may differ and NaN handling should be verified separately.