fix: SvelteMap incorrectly handles keys with undefined values

[MathiasWP]

Summary

SvelteMap had two bugs related to how it checked for key existence internally:

1. has() and get() returned wrong results for keys with undefined values

Both methods used super.get(key) !== undefined to determine if a key existed before creating a per-key reactive source. This fails for keys whose value is legitimately undefined, causing:

• has(key) to return false for existing keys with undefined values
• get(key) to skip creating a per-key source and fall back to tracking version, resulting in over-notification

Fix: Replace super.get(key) !== undefined with super.has(key) in both has() and get(), matching the pattern already used in SvelteSet.

2. delete() skipped reactive updates when a key had no per-key source

The size and version reactive updates were inside the if (s !== undefined) block, meaning they only fired when a per-key source existed (i.e., someone had previously called has() or get() on that specific key). If a key was added via the constructor or set() but never individually read, deleting it would not trigger reactive updates for effects depending on size or iterators.

Fix: Move set(this.#size, super.size) and increment(this.#version) to a separate if (res) block so they fire whenever a key is actually deleted, regardless of whether a per-key source existed.

Before fix

const map = new SvelteMap([['foo', undefined]]);
map.has('foo');  // false (should be true)
map.get('foo');  // undefined but tracks version instead of per-key source

After fix

const map = new SvelteMap([['foo', undefined]]);
map.has('foo');  // true
map.get('foo');  // undefined with correct per-key tracking

Test plan

Tests are in packages/svelte/src/reactivity/map.test.ts:

• map.has() returns true for constructor-initialized keys with undefined values
• map.get() returns undefined with proper per-key reactive tracking
• map.delete() triggers has()/get() reactivity for undefined-valued keys
• map.set(key, undefined) followed by has()/get() works correctly
• map.delete() triggers size reactivity for keys that were never individually read (no per-key source)
• All existing tests pass unchanged

🤖 Generated with Claude Code

[changeset-bot]

🦋 Changeset detected

Latest commit: 9f7abd4

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

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[7nik]

The test should be in packages/svelte/src/reactivity/map.test.ts

[Rich-Harris]

thank you!