perf(utils): rewrite defaults using spread for stable object shape

[spuppo-mux]

Refs #1335

Summary

defaults() is called once per setProps() call, which runs on every Lit update cycle for every component. During multi-player init it accumulates into hundreds of calls. The previous imperative for-in loop with dynamic property assignment (result[key as keyof T] = ...) produced result objects with unstable hidden classes in V8, slowing both the function itself and every downstream read of the resulting #props object. Switching to a spread-based functional pipeline preserves the documented contract (undefined values still filtered) while landing on V8's optimized spread fast path.

Changes

• defaults() rewritten as Object.fromEntries(...filter(...)) + spread merge
• Behavior unchanged: undefined values still skipped, defaults still fill in
• Existing 11-test suite passes without modification — the contract is identical, only the implementation strategy changed

Why this is faster

Two V8 mechanisms compound:

1. Hidden class stability — V8 compiles obj.label to a direct memory offset when the object's shape is predictable. Dynamic property assignment with a computed key (result[key] = value) forces shape transitions and pushes property reads into a slower polymorphic path. Object spread has a heavily optimized fast clone that produces a stable shape.
2. for-in prototype-chain check — for (const key in object) traverses the prototype chain by default. V8 has a fast path for plain objects but still verifies enumerability on every call. Object.entries only walks own enumerable keys.

The result objects from defaults() are read repeatedly afterwards (this.#props.label, this.#props.disabled, …), so the hidden-class effect compounds across every property access during init.

Impact

Measured on a 6 player multi-player sandbox html:

Before the changes:

After the changes - different scale, same task goes from ~110ms to ~30ms:

• LCP halves further
• One Long Task during init eliminated (no >50ms main-thread blocking task remains)

A Long Task disappearing means the engine got a yield point during init that previously didn't exist — paint can land at that yield, which is consistent with the additional LCP improvement.

Testing

pnpm -F @videojs/utils test src/object/tests/defaults.test.ts — existing tests pass without modification, including the cases that exercise the undefined-filtering contract.

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Note

Low Risk
Low risk refactor in a small utility that should preserve behavior; main risk is subtle differences in key enumeration/typing due to switching from for...in assignment to Object.entries + spread.

Overview
Refactors defaults() to build a defined object via Object.entries(...).filter(!isUndefined) and then return { ...defaultValues, ...defined } instead of imperatively copying and conditionally assigning properties.

The intent is to keep the same contract (ignore undefined inputs; preserve defined values) while producing a more stable object shape for performance.

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Cursor Bugbot has reviewed your changes and found 1 potential issue.

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Object.entries skips inherited reactive element properties

High Severity

Object.entries() only returns own enumerable properties, but ReactiveElement installs reactive properties as prototype accessors. The previous for...in loop correctly traversed the prototype chain, but the new code silently skips these properties. As a result, defaults() ignores user-set properties, always returning default values. This affects custom elements using setProps(this), making their property customization non-functional.

 

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[spuppo-mux]

Confirmed this is why this was much faster, it was skipping many attributes