Hacktoberfest 2026: the issues maintainers tagged for October, open and beginner-friendly. Browse Hacktoberfest issues

Suspense mode resumes with the previous observable's value on React 19

Open
  • Is something wrong?
#793 0 comments 0 reactions 0 assignees View on GitHub

Maintainers usually reply within 2 days

Nobody has claimed this yet.

Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
48/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Quiet
Tech stack
react, typescript
Domain
frontend

Research direction

Run npx vitest run useObservable and start with test/useObservable.test.tsx, especially emits the new observable's value if the observable is swapped out. Trace the named SuspenseSubject, preloadedObservables cache, and use-sync-external-store path. Done means the test is restored to plain it and passes under React 19 while retaining the React 18 behavior.

Written by the indexing model from the issue text.

Description

v5

useObservable in suspense mode suspends correctly when the observable and observableId are swapped, then resumes rendering the previous observable's value.

Found by the first test run ever executed against React 19 (#789); nothing had run against 19 before, only type-checks.

Reproduction

test/useObservable.test.tsx, emits the new observable's value if the observable is swapped out. The test renders a component under React.Suspense, emits Jeff on the first observable, swaps both the observable and the observableId, rerenders, then emits James on the second.

  • React 18.2.0: the fallback shows, then the component resumes with James. Passes.
  • React 19.2.8: the fallback shows (that assertion passes), then the component resumes with Jeff. Fails.

Deterministic: 3 runs each way, same @testing-library/react 16.3.2, only the React runtime swapped via npm install --no-save react@19 react-dom@19. No emulators involved, so npx vitest run useObservable reproduces it.

What is known and not known

  • The suspend half works. Only the resume is wrong, so this is not the known throw-vs-use() question: the 08-07 spike found the use() swap is a behavioural no-op on 19 and that React 19 emits no warning for the current throw form. Nothing here contradicts that.
  • The cause is not diagnosed. The plumbing involved (SuspenseSubject, the preloadedObservables cache keyed by observableId, and the use-sync-external-store shim delegating to React's own useSyncExternalStore on 19) overlaps what #790 covers, but no connection has been established and it should not be assumed.

Current state

#789's CI PR skips this test on React 19 only, so the new test job can land green. The skip is a ? it.skip : it ternary rather than it.skipIf, because these globals are typed by @types/jest and skipIf fails tsc while working at runtime.

Restore it to a plain it when this is fixed.

Dominant language
TypeScript
Stars
3.6k
Forks
403
Avg merge
4d 19h
Merged PRs (30d)
11

Getting set up

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

More from FirebaseExtended/reactfire

All issues in FirebaseExtended/reactfire

Similar issues

More TypeScript issues

Get new issues in your inbox

A short digest of beginner-friendly GitHub issues.