Add WebAssembly.instantiateStreaming polyfill for Node.js ESM context

[Copilot]

WASM-based Prettier plugins (e.g., assemblyscript-prettier) fail in v12.1.0+ with WebAssembly.instantiateStreaming is not a function. The ESM migration in v12.1.0 introduced dynamic import() with file URLs, which creates a module context where Node.js doesn't expose WebAssembly.instantiateStreaming.

Changes

• Added polyfill in PrettierDynamicInstance.ts before dynamic imports
• Polyfill provides WebAssembly.instantiateStreaming using WebAssembly.instantiate + arrayBuffer() fallback
• Applied globally once before loading Prettier modules
• Added comprehensive test suite in wasm-polyfill.test.ts to verify polyfill behavior

function polyfillWebAssemblyInstantiateStreaming(): void {
  const wasm = (globalThis as any).WebAssembly;
  if (typeof wasm !== "undefined" && !wasm.instantiateStreaming) {
    wasm.instantiateStreaming = async function (source, importObject) {
      const response = await source;
      const bytes = await response.arrayBuffer();
      return wasm.instantiate(bytes, importObject);
    };
  }
}

Testing

• ✅ Polyfill is applied when WebAssembly.instantiateStreaming is missing
• ✅ Existing implementations are not overridden
• ✅ Polyfilled function correctly uses WebAssembly.instantiate with arrayBuffer()

Non-WASM plugins unaffected. No behavioral changes to existing functionality.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>v12.1.0 break some plugin</issue_title>
<issue_description>### Preflight Checklist

• I have read the troubleshooting guide
• I have searched existing issues and this is not a duplicate
• This is a bug with the VS Code extension, not with how Prettier formats code

Issue Summary

after upgrade to v12.1.0, plugin based on wasm does not work anymore

["ERROR" - 08:41:43] WebAssembly.instantiateStreaming is not a function
TypeError: WebAssembly.instantiateStreaming is not a function
	at file:///Users/caicongcong/dev/warpo/node_modules/assemblyscript-prettier/node_modules/assemblyscript/dist/assemblyscript.js:9:38049

Reproduction Repository

https://github.com/wasm-ecosystem/warpo

Steps to Reproduce

1. clone the repo
2. change some format in assemblyscript/src/...
3. format with plugin

Expected Behavior

plugin with wasm can be load successfully

Actual Behavior

error

Operating System

macOS

VS Code Version

Version: 1.108.1 Commit: 585eba7c0c34fd6b30faac7c62a42050bfbc0086 Date: 2026-01-14T14:55:44.241Z Electron: 39.2.7 ElectronBuildId: 12953945 Chromium: 142.0.7444.235 Node.js: 22.21.1 V8: 14.2.231.21-electron.0 OS: Darwin arm64 25.2.0

Prettier Extension Version

v12.1.0

Prettier Version

3.6.2

Prettier Extension Logs

["ERROR" - 08:41:43] WebAssembly.instantiateStreaming is not a function
TypeError: WebAssembly.instantiateStreaming is not a function
	at file:///Users/caicongcong/dev/warpo/node_modules/assemblyscript-prettier/node_modules/assemblyscript/dist/assemblyscript.js:9:38049

Prettier Configuration

printWidth: 120
tabWidth: 2
useTabs: false
semi: true
singleQuote: false
quoteProps: "as-needed"
jsxSingleQuote: false
trailingComma: "es5"
bracketSpacing: true
arrowParens: "always"
vueIndentScriptAndStyle: false
endOfLine: "lf"
plugins:
  - assemblyscript-prettier
```</issue_description>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes v12.1.0 break some plugin #3907

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[ntotten]

@copilot we need a test case for this

[Copilot]

@copilot we need a test case for this

Added test case in commit 393f4c3. The test file wasm-polyfill.test.ts includes three test cases that verify:

1. The polyfill is applied when WebAssembly.instantiateStreaming is missing
2. Existing implementations are not overridden
3. The polyfilled function correctly uses WebAssembly.instantiate with arrayBuffer()