## Context In !226171, the `UPDATE_NOTE` mutation in `app/assets/javascripts/notes/store/legacy_notes/mutations.js` was changed from using `splice` (full object replacement) to delegating to `useDiscussions().updateNote()`, which uses lodash `merge` (deep merge into the existing object reference). This was done to preserve reactive properties (`isEditing`, `editedNote`) that were being lost when `splice` replaced note objects wholesale. ## Concern `splice` and `merge` have different semantics for property removal: - **`splice`** replaced the entire note object. If the server response omitted a property that was previously present, that property was dropped. - **`merge`** deep-merges into the existing object. If the server response omits a property, the stale value is preserved on the existing object. This is not currently causing any known bugs, but could be a subtle source of issues in the future if the API response shape for notes ever changes to intentionally omit previously-present properties. An alternative like `Object.assign(existingNote, note)` would do a shallow replacement of all top-level keys on the same object reference (preserving reactive props added at the top level) while more closely matching the old replacement semantics. ## Original discussion - [ ] @thomasrandolph started a [discussion](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/226171#note_3139596407): > The old `splice` did full object replacement; `merge` does deep merge. If the server response ever omits a property that was previously present on a note, `splice` would have dropped it, but `merge` preserves the stale value on the existing object. Would something like `Object.assign(existingNote, note)` better match the old replacement semantics while still keeping the same object reference (and its reactive props)? > > |:grey_question: [What is this](https://gitlab.com/thomasrandolph/reviews/wikis/Review-Metadata-Table)|| > |-|-| > | Type | **Question** | > | Blocker | No | > | Tags | _correctness_, _semantics_ | > | Concern Level 1-10 | 3 |