test(middleware/immer): add runtime tests for immer middleware

[KimHyeongRae0]

Related Bug Reports or Discussions

N/A — coverage-only test additions. No source changes.

Summary

The immer middleware (src/middleware/immer.ts) has no dedicated runtime test file. tests/middlewareTypes.test.tsx only exercises it at the type level, leaving the store.setState override (lines 77–83) entirely uncovered at runtime. Other middleware (devtools, persist, subscribeWithSelector) already have dedicated runtime test files.

This mirrors the pattern accepted in #3442 (vanilla had subscribe coverage, React did not → add the missing runtime tests).

Tests added (tests/immer.test.tsx)

• should mutate state via an immer draft (function updater) — covers the produce(updater) path
• should merge state when given a non-function updater — covers the : updater path
• should replace the entire state when replace flag is true — covers replace flag propagation
• should produce nested state updates via a draft without mutating the previous state — verifies immer's structural sharing
• should work with vanilla createStore — covers the vanilla surface

Coverage delta for src/middleware/immer.ts

	Before	After
Statements	71.42%	100%
Branches	0%	100%
Functions	66.66%	100%
Lines	66.66%	100%

Suite total: 207 → 212 tests, all passing.

Check List

• pnpm run fix for formatting and linting code and docs

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Project	Deployment	Actions	Updated (UTC)
zustand-demo	Ready	Preview, Comment	Apr 21, 2026 2:04am

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[pkg-pr-new]

• demo

• starter

  npm i https://pkg.pr.new/zustand@3471
  

commit: 62a421f

[dai-shi]

@sukvvon Would you like to give a review?

[sukvvon]

Suggested change

	it('should merge state when given a non-function updater', () => {
	it('should not apply produce when updater is not a function', () => {

The name "merge" feels slightly misleading here — merging is actually vanilla setState's behavior, while what this test truly covers is the right-hand branch at immer.ts:79, where a non-function updater bypasses produce. Something like this would make the intent clearer.

[KimHyeongRae0]

Thanks for the catch — you're right that "merge" conflates this with vanilla setState semantics, when the test is really about the non-function branch at immer.ts:79 bypassing produce. Renamed to should not apply produce when updater is not a function in 62a421f.

[sukvvon]

@dai-shi Checked out and verified locally — immer.ts coverage goes 66% → 100% and tests 207 → 212 exactly as described. The should produce nested state updates via a draft without mutating the previous state case nicely covers immer's structural sharing. Looks good to merge.

[dai-shi]

Thanks @KimHyeongRae0 @sukvvon.

[KimHyeongRae0]

Thanks @dai-shi for the quick merge, and @sukvvon for taking the time to check it out locally — really appreciate the thoughtful rename suggestion and the coverage verification.