Crash in valueSetter: animation.onStart is undefined (useDerivedValue) #8974
Description
Description
We're seeing a crash in production that traces back to valueSetter.ts line 42, where animation.onStart(...) is called without checking that onStart actually exists on the animation object.
The guard on lines 16-21 checks for value.onFrame !== undefined, but never checks onStart. So if an object enters the animation path that has onFrame but is missing onStart, it blows up with "undefined is not a function".
From our Sentry data this is happening inside the useDerivedValue mapper path — the minified stack looks like:
initializeAnimation → valueSetter → set value → useDerivedValue updater → mapperRun
It's not tied to any specific screen or component. We see it across the whole app, on all iOS versions (18.x through 26.x), all device models.
We've been on 4.1.6 and I checked the source in 4.2.0 and 4.2.1 as well — valueSetter.ts is identical in all three, so the bug is still present.
The bug
In src/valueSetter.ts:
// line 16-21: guard only checks onFrame
if (
typeof value === 'function' ||
(value !== null &&
typeof value === 'object' &&
(value as unknown as AnimationObject).onFrame !== undefined)
) {
// ...
// line 42: calls onStart without checking it exists
const initializeAnimation = (timestamp: number) => {
animation.onStart(animation, mutable.value, timestamp, previousAnimation);
};If onFrame is defined but onStart is not, the code enters the animation branch and then crashes on animation.onStart(...).
Suggested fix
Add a guard before calling initializeAnimation:
if (!animation.onStart || !animation.onFrame) {
mutable._value = animation.current ?? value;
return;
}
const initializeAnimation = (timestamp: number) => {
animation.onStart(animation, mutable.value, timestamp, previousAnimation);
};Or at minimum, check onStart alongside onFrame in the initial guard on line 16-21.
Steps to reproduce
Hard to give a minimal repro since it happens intermittently in production across many screens. The common thread is useDerivedValue returning objects that end up triggering the animation code path. We've confirmed it's not specific to any particular derived value — it happens app-wide.
Snack or a link to a repository
N/A — production-only crash, but the bug is clearly visible in the source.
Reanimated version
4.1.6 (also verified source is identical in 4.2.0, 4.2.1)
React Native version
0.79.2
Platforms
iOS
JavaScript runtime
Hermes
Workflow
Expo
Architecture
New Architecture (Fabric)
Build type
Release
Device
Real device (all models)
Device model
All — iPhone 12 through 16 Pro Max
Acknowledgements
- I searched for similar issues in the repository and found none that match
- I understand that the team needs a minimal reproducible scenario