Bug: useEffect does not run after changed dependencies due to startTransition

[daniele-orlando]

React version: 19.1.0

useEffect does not run after changed dependencies, due to startTransition.

Steps To Reproduce

1. Open the StackBlitz reproduction
2. open the terminal
3. npm install
4. npm run start:dev
5. Open the browser Developer Tools
6. Select Console tab from the Developer Tools
7. Load/reload the StackBlitz preview page
8. Watch the Developer Tools console logs.

Link to code reproductions:

• StackBlitz
• react-19.1-use-effect-bug.zip

The current behavior

File src/main.hooks.ts contains an hook named useData. Inside this hook, the useEffect at line 53 should be executed 2 times because dependencies change 2 times (first with value undefined and second time with value 1), but it is executed only once.

Following Developer Tools Console screenshot shows the issue.

The expected behavior

The useEffect at line 53 should be executed 2 times because dependencies change 2 times (first with value undefined and second time with value 1).

Following Developer Tools Console screenshot shows the correct behavior.

Correct behavior can be achieved with one of these two workarounds:

• comment line 32 disabling the setExample(ExampleInitialValue)
• de-comment line 38 enabling setExample(ExampleInitialValue + 1)

More Details

In this screenshot I show a brief explanation of the hook flow.

[eps1lon]

The repro doesn't contain any code. Can you reduce this as much as possible without any 3rd party code (e.g. @downforce/*)? You can inline the 3rd party code and remove as much as possible. Otherwise we won't be able to help with this issue.

[daniele-orlando]

Hi dear @eps1lon and thanks for the reply.

Sorry for the snag with the StackBlitz project; I've updated the project and now it should work properly.

Sure I can inline the @downforce library and I'll try to remove as much code as possible from it. I ping you later when I've done.

[daniele-orlando]

Hi @eps1lon, sorry for the delay, it has been a busy week.

I've updated the StackBlitz project and the zip archive as requested, removing any external dependency and inlining relevant code.

You should be able to reproduce the bug at this point.

[eps1lon]

That's a lot of code to unpack. Especially with Redux still in there which I suspect might be using useSyncExternalStore. Can you condense this to use less abstractions and indirections? Ideally it's all just one file.

[daniele-orlando]

It's a Redux-like implementation, but internally it does not use useSyncExternalStore as you can see (there is no import of useSyncExternalStore).

Following your request, I tried to re-implement a simpler pseudo code, but of course I changed the actual code flow, and in fact I have not managed to let the bug happen.
I don’t know the exact code path and states/transitions combinations leading to the bug, so I don't know if it feasible to simplify it.

Given the fact that half of the code is TypeScript types, and the other half is simple pure functions and basic hooks (useState, useEffect, useRef, useMemo) and nothing weird is happening, can you confirm the bug and promote it for further investigation?

[daniele-orlando]

I've updated the reproduction code (StackBlitz and zip file) removing all unneeded code. The remaining vendor code is the bare minimum needed to make it work.