fix(reactivity): handle objects with custom Symbol.toStringTag (fix #10483)

[ATOM00blue]

Fixes #10483

Problem

reactive() / ref() silently fail to make an object reactive when that object (or a subclass of a built-in collection) defines a custom Symbol.toStringTag.

getTargetType decides whether a value should be wrapped in a proxy based on toRawType(value), which derives the type from Object.prototype.toString.call(value). When a custom Symbol.toStringTag is present, that call returns e.g. "[object Goat]" instead of "[object Object]", so toRawType returns "Goat", targetTypeMap falls through to INVALID, and the value is returned untouched (not reactive).

This is a real-world issue because such tags can come from third-party libraries, where the consumer cannot remove them. The object is mutated correctly but the UI never updates.

const original = { [Symbol.toStringTag]: 'Goat', foo: 1 }
const state = reactive(original)
isReactive(state) // false  (expected: true)

The same applies to subclasses of Map/Set that expose a custom tag, which are detected as INVALID rather than COLLECTION.

Fix

When toRawType does not match one of the known built-in types, fall back to prototype-based checks (instanceof for collections/arrays, prototype comparison for plain objects) to determine the correct TargetType. The fast path for normal objects/arrays/collections is unchanged, so there is no overhead for the common case.

Tests

Added cases in reactive.spec.ts covering a plain object with a custom Symbol.toStringTag, and Map/Array subclasses that define a tag. They fail on main and pass with this change.

Summary by CodeRabbit

• Bug Fixes
  • Improved reactivity handling to ensure consistent detection of Map, Set, Array, and plain objects.

[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: 54241742-95de-4f6b-8a7c-a9a94ab5f60f

📥 Commits

Reviewing files that changed from the base of the PR and between bbdf86d and c692316.

📒 Files selected for processing (2)

• packages/reactivity/__tests__/reactive.spec.ts
• packages/reactivity/src/reactive.ts

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📝 Walkthrough

Walkthrough

Fixed type detection in reactive() by refactoring targetTypeMap to accept the full target value and apply instanceof/prototype fallback checks, allowing correct reactivity for objects with custom Symbol.toStringTag. Added tests confirming reactive behavior for such objects and collections.

Changes

Symbol.toStringTag Detection Fix

Layer / File(s)	Summary
Target type detection with Symbol.toStringTag fallback packages/reactivity/src/reactive.ts	targetTypeMap signature changed to accept the full Target value instead of a precomputed raw type string. Implementation now calls toRawType(value) internally and applies instanceof checks and prototype-based isPlainObject helper as fallback to correctly identify built-in collections (Map, Set, WeakMap, WeakSet) and Array/plain objects even when custom Symbol.toStringTag would mask the actual type. getTargetType updated to pass the full value to targetTypeMap.
Tests for reactive behavior with Symbol.toStringTag packages/reactivity/__tests__/reactive.spec.ts	Added two test cases verifying reactive() correctly observes objects with custom Symbol.toStringTag and collection subtypes (Map, Array) with custom Symbol.toStringTag. Tests confirm reactive status and that effects respond to mutations (property access, Map.has with set, and Array.length with push).

🎯 2 (Simple) | ⏱️ ~12 minutes

Suggested labels

ready to merge, scope: reactivity

Poem

A rabbit hops through type detection's maze,
Where Symbol.toStringTag once caused a daze,
Now fallback checks with instanceof sight,
Make proxies reactive—all's working 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 clearly and specifically describes the main change: fixing reactivity for objects with custom Symbol.toStringTag, referenced to issue #10483.
Linked Issues check	✅ Passed	The PR implementation fully addresses issue #10483 by adding fallback prototype-based checks to correctly detect built-in types when custom Symbol.toStringTag confuses toRawType(), and includes test cases verifying the fix works.
Out of Scope Changes check	✅ Passed	All changes are directly scoped to fixing the Symbol.toStringTag detection issue: test cases covering the fix scenario and implementation changes to the type detection logic in reactive.ts.

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✨ Finishing Touches

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[Copilot]

Pull request overview

This PR fixes a reactivity edge case where reactive() / ref() fail to proxy values that define a custom Symbol.toStringTag, causing toRawType()-based target classification to mis-detect the value as INVALID.

Changes:

• Update target-type detection to fall back to prototype/instanceof checks when toRawType() returns an unknown type (custom Symbol.toStringTag).
• Add tests covering a plain object with a custom Symbol.toStringTag, plus Map/Array subclasses with custom tags.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
packages/reactivity/src/reactive.ts	Adds fallback target-type detection when toRawType() is affected by custom Symbol.toStringTag.
packages/reactivity/tests/reactive.spec.ts	Adds regression tests for custom Symbol.toStringTag on objects and collection/array subtypes.

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commit: c692316

[edison1105]

use isArray instead.