fix(reactivity): trigger immediate watch for empty sources

[leno23]

Fixes #14898.\n\nWhen watching an empty array of sources with immediate: true, the multi-source change check used Array#some(), which never passes for an empty array. Treat the first immediate job as a callback trigger so empty multi-source watchers follow the same immediate semantics as other sources.\n\nTest: pnpm vitest packages/runtime-core/tests/apiWatch.spec.ts --run

Summary by CodeRabbit

• Bug Fixes
  • Fixed the watch API to properly invoke callbacks when the immediate option is enabled, ensuring callbacks execute as expected during initial setup.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 7afadd80-f4f4-4c35-a0f4-81fecde4c86a

📥 Commits

Reviewing files that changed from the base of the PR and between 9d92dbd and a426cd3.

📒 Files selected for processing (2)

• packages/reactivity/src/watch.ts
• packages/runtime-core/__tests__/apiWatch.spec.ts

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📝 Walkthrough

Walkthrough

The watch callback now runs immediately when immediate: true even if the source array is empty or no change is detected on the first execution. The job condition checks immediateFirstRun to invoke the callback during the initial scheduled run, and a test validates this behavior.

Changes

Watch immediate execution fix

Layer / File(s)	Summary
Job condition update and immediate execution test packages/reactivity/src/watch.ts, packages/runtime-core/__tests__/apiWatch.spec.ts	The watch job callback's decision condition now includes immediateFirstRun to ensure the callback executes on immediate-first scheduled execution even when change-detection would skip it. A test verifies that watch([], cb, { immediate: true }) invokes the callback with empty arrays.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

Possibly related PRs

• vuejs/core#12296: Both PRs modify the watch() job logic that governs when the callback (cb) is executed and what value state it observes.

Suggested labels

ready to merge, scope: reactivity, :hammer: p3-minor-bug

Poem

🐰 An empty array now watches bright,
With immediate: true, it runs just right,
No changes needed, the callback fires,
Empty sources grant their desires,
Variables fade, but semantics stay right!

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately and specifically describes the main fix: triggering immediate watch callbacks for empty source arrays.
Linked Issues check	✅ Passed	The code changes directly address the requirement from issue #14898: enabling immediate callbacks to run when watching empty source arrays.
Out of Scope Changes check	✅ Passed	All changes are narrowly scoped to fixing the immediate watch behavior for empty sources and include a corresponding test case.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[github-actions]

Size Report

Bundles

File	Size	Gzip	Brotli
runtime-dom.global.prod.js	106 kB (+3 B)	40.1 kB (+5 B)	36 kB (-2 B)
vue.global.prod.js	164 kB (+3 B)	60.1 kB (+9 B)	53.5 kB (-31 B)

Usages

Name	Size	Gzip	Brotli
createApp (CAPI only)	48.8 kB	19 kB	17.4 kB
createApp	56.9 kB (+3 B)	22 kB (+9 B)	20.1 kB (+14 B)
createSSRApp	61.2 kB (+3 B)	23.8 kB (+10 B)	21.7 kB (+3 B)
defineCustomElement	63.1 kB (+3 B)	23.9 kB (+9 B)	21.8 kB (+3 B)
overall	71.7 kB (+3 B)	27.4 kB (+4 B)	25 kB (+8 B)

[pkg-pr-new]

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@vue/compiler-core

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@vue/reactivity

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@vue/runtime-core

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@vue/runtime-dom

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vue

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commit: a426cd3

[edison1105]

duplicate of #14914