TextEncoder.encodeInto('') returns the previous call's { read, written } #2173

Describe the bug

TextEncoder.prototype.encodeInto('') returns the result of the previous encodeInto call, from any encoder instance, instead of { read: 0, written: 0 }. Nothing is written to the destination, but callers that trust written read stale bytes.

We hit it through Yuku's WebAssembly parser (@yuku-core/wasm), which TanStack Router/Start is moving to: parsing an empty string parses leftover bytes, so Start server functions fail in WebContainers.

Link to the blitz that caused the error

https://stackblitz.com/github/Sheraff/webcontainer-encodeinto-repro

Steps to reproduce

Open the project. It runs npm start (index.js):

const a = new TextEncoder()
const b = new TextEncoder()
a.encodeInto('abc', new Uint8Array(9)) // { read: 3, written: 3 }
a.encodeInto('', new Uint8Array(4))    // { read: 3, written: 3 }
b.encodeInto('', new Uint8Array(4))    // { read: 3, written: 3 }

and prints BUG: encodeInto("") returns the previous call's { read, written } (WebContainer, Node v22.22.3).

Expected behavior

Both empty calls return { read: 0, written: 0 }, as in Node (v22.12.0, v24.8.0), where the project prints OK.

Parity with Local

• I have run the project in my local machine and I could not reproduce the issue.

Platform

• OS: Linux
• Browser: Chromium (Playwright)
• WebContainer: process.versions.webcontainer = 2ed8f36d

Additional context

Found while testing TanStack Router's Yuku migration in the tanstack.com example playground. Written with Claude Code (AI-assisted).