structuredClone shortens sparse arrays with trailing holes

Description

The React Native structuredClone polyfill initializes an array clone as [] and only assigns enumerable keys. If a sparse array's highest populated index precedes its declared length, the trailing holes are lost and the clone is silently shortened. For example, new Array(3) with only index 1 populated becomes length 2.

Expected behavior

The clone should preserve the source array's declared length and holes. Node's native structuredClone follows this behavior.

React Native Version

0.87.1 and current main

Affected Platforms

JavaScript runtime - All

まず React Native の structuredClone polyfill を見つけ、index 1 だけに値が設定された new Array(3) のような疎な配列で問題を再現します。クローンがソースの長さと末尾のホールを保持することを、Node のネイティブな structuredClone の動作と照合して確認します。