Memoize shallowEqual verdicts by object identity in useOnyx

[TMisiukiewicz]

Details

Problem
When a large single Onyx key updates (e.g. personalDetailsList with ~18k members), every subscribed useOnyx hook independently runs shallowEqual(previousValue, newValue) inside getSnapshot() to decide whether to re-render. All of these hooks are comparing the exact same two object references (the old and new cache-owned values), yet each pays for its own full O(keys) walk.

Measured on a large account with ~200 subscribers to personalDetailsList: a single update triggered 29 full walks, ~512,000 key comparisons, ~530ms of synchronously blocked JS thread.

Fix
Memoize shallowEqual verdicts by the identity of the compared object pair:

WeakMap>

The first hook comparing a given (prev, next) pair pays for the real shallowEqual walk and stores the verdict; every other hook comparing the same pair resolves in O(1). Non-object inputs bypass the memo (they're O(1) to compare anyway).

Why this is safe

• No behavior change. The memo caches a pure function keyed by its exact inputs — same semantics, same loading states, same re-render suppression. Cases where shallowEqual legitimately returns true on different references (fresh collection snapshots with unchanged members) behave identically.
• No invalidation logic. A verdict can only be read by holding both exact object references, so a stale answer is unreachable by construction. When Onyx replaces a value, the old object — and its verdict entries — are garbage-collected together (WeakMap).

Results

	Before	After
shallowEqual time per personalDetailsList update	~530ms	~17ms
Full walks per update	29	1
Key comparisons per update	~512k	~18k
Behavior change	—	none

Scope note
This change removes the redundancy (N hooks paying for N identical comparisons), not the existence of the O(members) comparison — one full walk per update remains, and it is inherent to personalDetailsList being a single monolithic key. The root-cause fix is migrating it to a proper collection (personalDetails_), so an update only ever touches the changed member's subscribers. That is a large App/backend migration and is intentionally out of scope here; this optimization stands on its own regardless, since it applies to every large key with multiple subscribers (report_, policy_, snapshots, derived keys), not just this one.

Related Issues

$ Expensify/App#96856

Linked E/App PR

Expensify/App#96620

Automated Tests

Manual Tests

1. Navigate through multiple cached/not cached reports in the LHN
2. Verify messages, avatars and display names render correctly
3. Go to Account -> Profile and change the display name
4. Go back to LHN and verify the new name appears in report headers, LHN and message autorship without reload

Author Checklist

• I linked the correct issue in the ### Related Issues section above
• I wrote clear testing steps that cover the changes made in this PR
  • I added steps for local testing in the Tests section
  • I tested this PR with a High Traffic account against the staging or production API to ensure there are no regressions (e.g. long loading states that impact usability).
• I included screenshots or videos for tests on all platforms
• I ran the tests on all platforms & verified they passed on:
  • Android / native
  • Android / Chrome
  • iOS / native
  • iOS / Safari
  • MacOS / Chrome / Safari
  • MacOS / Desktop
• I verified there are no console errors (if there's a console error not related to the PR, report it or open an issue for it to be fixed)
• I followed proper code patterns (see Reviewing the code)
  • I verified that any callback methods that were added or modified are named for what the method does and never what callback they handle (i.e. toggleReport and not onIconClick)
  • I verified that the left part of a conditional rendering a React component is a boolean and NOT a string, e.g. myBool && <MyComponent />.
  • I verified that comments were added to code that is not self explanatory
  • I verified that any new or modified comments were clear, correct English, and explained "why" the code was doing something instead of only explaining "what" the code was doing.
  • I verified proper file naming conventions were followed for any new files or renamed files. All non-platform specific files are named after what they export and are not named "index.js". All platform-specific files are named for the platform the code supports as outlined in the README.
  • I verified the JSDocs style guidelines (in STYLE.md) were followed
• If a new code pattern is added I verified it was agreed to be used by multiple Expensify engineers
• I followed the guidelines as stated in the Review Guidelines
• I tested other components that can be impacted by my changes (i.e. if the PR modifies a shared library or component like Avatar, I verified the components using Avatar are working as expected)
• I verified all code is DRY (the PR doesn't include any logic written more than once, with the exception of tests)
• I verified any variables that can be defined as constants (ie. in CONST.js or at the top of the file that uses the constant) are defined as such
• I verified that if a function's arguments changed that all usages have also been updated correctly
• If a new component is created I verified that:
  • A similar component doesn't exist in the codebase
  • All props are defined accurately and each prop has a /** comment above it */
  • The file is named correctly
  • The component has a clear name that is non-ambiguous and the purpose of the component can be inferred from the name alone
  • The only data being stored in the state is data necessary for rendering and nothing else
  • If we are not using the full Onyx data that we loaded, I've added the proper selector in order to ensure the component only re-renders when the data it is using changes
  • For Class Components, any internal methods passed to components event handlers are bound to this properly so there are no scoping issues (i.e. for onClick={this.submit} the method this.submit should be bound to this in the constructor)
  • Any internal methods bound to this are necessary to be bound (i.e. avoid this.submit = this.submit.bind(this); if this.submit is never passed to a component event handler like onClick)
  • All JSX used for rendering exists in the render method
  • The component has the minimum amount of code necessary for its purpose, and it is broken down into smaller components in order to separate concerns and functions
• If any new file was added I verified that:
  • The file has a description of what it does and/or why is needed at the top of the file if the code is not self explanatory
• If the PR modifies a generic component, I tested and verified that those changes do not break usages of that component in the rest of the App (i.e. if a shared library or component like Avatar is modified, I verified that Avatar is working as expected in all cases)
• If the main branch was merged into this PR after a review, I tested again and verified the outcome was still expected according to the Test steps.
• I have checked off every checkbox in the PR author checklist, including those that don't apply to this PR.

Screenshots/Videos

Android: Native

Android: mWeb Chrome

iOS: Native

iOS: mWeb Safari

MacOS: Chrome / Safari

Screen.Recording.2026-07-22.at.08.57.35.mov

[youssef-lr]

Nice!

[os-botify]

🚀 Published to npm in 3.0.93 🎉