[kinyoklion]

Summary

Flag keys such as constructor, toString or __proto__ are ordinary keys, but some code kept them in plain objects:

• The event summarizer threw when it summarized an evaluation of a flag whose key is a built-in object member, so variation() failed for that flag in the server and client SDKs. It now uses maps, and builds the summary on a null-prototype object so that a flag named __proto__ stays in the summary.
• allFlagsState() dropped a flag named __proto__ from allValues() and toJSON(), and getFlagValue() could return an inherited property for a key that is not in the state. The state still serializes to plain objects, which React server components require.
• The in-memory store dropped a flag named __proto__ from all() and when it applied an update, and the FDv2 payload conversion dropped it unless it was the first item of its kind, so the flag was not found. Both now use null-prototype objects, as the override layer already does. The in-memory store's all() result now has no prototype, like the override layer's.

New unit tests cover each case. The node-server contract tests pass new server-side evaluation tests for these keys with FDv1 and FDv2.

_{Opened from termag}

---

Note

Overview
Fixes incorrect handling when feature flag keys are JavaScript built-in names such as __proto__, constructor, or toString. Assigning those keys on normal objects can mutate the prototype or hide entries, which broke summarization, bootstrap state, the in-memory store, and FDv2 payload conversion.

Event summarization now keeps counters in Map instances and builds the summary features object with Object.create(null), so every flag key is summarized and survives JSON.stringify.

FlagsStateBuilder stores values and metadata via Object.defineProperty, uses hasOwnProperty for lookups, and still emits plain objects from toJSON() for frameworks like React server components.

In-memory feature store and FDv2 payload listener use null-prototype maps when listing, snapshotting, merging, and converting updates so flags like __proto__ are retained even when they are not the first item in a batch.

New unit tests cover these paths across EventSummarizer, FlagsStateBuilder, InMemoryFeatureStore, and createPayloadListenerFDv2.

^{Reviewed by Cursor Bugbot for commit b67ad28. Bugbot is set up for automated code reviews on this repo. Configure here.}

[github-actions]

@launchdarkly/js-sdk-common size report
This is the brotli compressed size of the ESM build.
Compressed size: 32497 bytes
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Uncompressed size: 153345 bytes

[github-actions]

@launchdarkly/js-client-sdk size report
This is the brotli compressed size of the ESM build.
Compressed size: 34545 bytes
Compressed size limit: 35000
Uncompressed size: 123262 bytes

[github-actions]

@launchdarkly/js-client-sdk-common size report
This is the brotli compressed size of the ESM build.
Compressed size: 26066 bytes
Compressed size limit: 44000
Uncompressed size: 168326 bytes