Cannot convert undefined value to object, js engine: hermes

Setting enableHermes to true would cause Cannot convert undefined value to object, js engine: hermes.

Confirmed it by setting the value to false and rebuilding.

React Native version: System: OS: Linux 5.0 Antergos Linux undefined CPU: (8) x64 Intel® Core™ i7-2600 CPU @ 3.40GHz Memory: 2.91 GB / 7.77 GB Shell: 5.7.1 - /usr/bin/zsh Binaries: Node: 11.14.0 - /usr/bin/node Yarn: 1.16.0 - /usr/bin/yarn npm: 6.9.0 - /usr/bin/npm SDKs: Android SDK: API Levels: 28 Build Tools: 28.0.3 IDEs: Android Studio: 3.3 npmPackages: react: 16.8.6 => 16.8.6 react-native: 0.60.3 => 0.60.3

Steps To Reproduce

1. Follow upgrade guide from 0.60.0 to 0.60.3
2. enable enableHermes in app/build.gradle
3. ./gradlew clean
4. react-native run-android

Describe what you expected to happen: Should work just fine as before.

[rszalski]

having same error, any idea?

@hackdie this is probably not a Hermes-related error but rather a normal JS error. The string js engine: hermes is there just to inform you that you’ve enabled Hermes correctly, and is not part of the actual error text. But it can be confusing at first.

If debugging in chrome doesn’t yield an answer, there is a blunt way of debugging this: try doing a binary search on your components (meaning, comment half of them out, see if the error is there, if not, comment the other half out, etc.) to pinpoint where this originates. It’s possible that you are just passing an undefined where an object is expected.

There might be something that makes Hermes more susceptible to this issue (which would give the perception of being caused by Hermes), but I don’t know the internals, just a thought.

[rszalski]

@xxRockOnxx Try debugging JS in Chrome debugger and enable „Pause on exceptions”. This way you should be able to at least see where the problem originates. It would be good to have a minimal reproducible example to work with.

I had the same issue but it turned out that it wasn’t actually Hermes (actually it was Reactotron trying to notify me of subscriptions to an incorrectly configured Redux store). I’m mentioning this because I also though that disabling Hermes fixed the issue, but it was only coincidentally related and stateful.