nextjs - 💡(How to fix) Fix Turbopack disables PropertyLiterals and MemberExpressionLiterals ES3 transforms, breaking Safari 15 strict mode [1 comments, 2 participants]

Link to the code that reproduces this issue

https://github.com/veniceai/turbopack-safari-reserved-words-repro

To Reproduce

1. Clone the repo and install dependencies:
   git clone https://github.com/veniceai/turbopack-safari-reserved-words-repro cd turbopack-safari-reserved-words-repro
   npm install

2. Build with Turbopack: npm run build

3. Verify the output chunks contain unquoted reserved words: npm run verify (This finds 7 violations: {const:, let:, var:} as unquoted object keys in AJV chunks)

4. Start the production server and open in Safari 15 / iOS 15 (e.g. via BrowserStack): npm start Open http://localhost:3000

5. Safari 15 console shows: SyntaxError: Unexpected keyword 'var'. Expected '}' to end an object literal.

The bug is in turbopack-ecmascript/src/transform/mod.rs — PropertyLiterals and MemberExpressionLiterals are excluded from preset_env alongside ReservedWords. Only ReservedWords needs to be excluded (it breaks module resolution). The other two are safe and are needed to quote reserved-word property names for Safari 15.

Current vs. Expected behavior

● ## Current behavior

Turbopack production builds (next build --turbo) emit unquoted reserved words
as object property names in JS chunks. For example, AJV's codegen/scope.js:

exports.varKinds = {const: new Name("const"), let: new Name("let"), var: new Name("var")}

Safari 15 / iOS 15 parses ES modules in strict mode, and its parser incorrectly rejects unquoted reserved words as property names with:

SyntaxError: Unexpected keyword 'var'. Expected '}' to end an object literal.
SyntaxError: Unexpected keyword 'let'. Cannot use 'let' as an identifier name for a LexicalDeclaration.

This affects any dependency that uses reserved words as object keys. Common ones include ajv, @chakra-ui/styled-system, framer-motion, and mingo.

The root cause is in turbopack-ecmascript/src/transform/mod.rs where three ES3 transforms are excluded from preset_env:

exclude: vec![
    FeatureOrModule::Feature(Feature::ReservedWords),
    FeatureOrModule::Feature(Feature::MemberExpressionLiterals),
    FeatureOrModule::Feature(Feature::PropertyLiterals),
]

ReservedWords must be excluded (it breaks Turbopack's module resolution), but PropertyLiterals and MemberExpressionLiterals are safe — they only affect property name quoting, not identifier resolution.

Expected behavior

Turbopack should quote reserved-word property names in output chunks:

exports.varKinds = {"const": new Name("const"), "let": new Name("let"), "var": new Name("var")}

This is what webpack/Babel builds produce via @babel/plugin-transform-property-literals, and what SWC's swc_ecma_compat_es3::prop_lits transform does when enabled.

The fix is to only exclude ReservedWords from preset_env, leaving PropertyLiterals and MemberExpressionLiterals enabled:

exclude: vec![
    FeatureOrModule::Feature(Feature::ReservedWords),
]

iOS 15 / Safari 15 still has ~5-8% mobile market share. All Turbopack production builds are broken for these users if any dependency uses reserved words as property names.

Provide environment information

Operating System:                                                                                                                                                                                              
    Platform: linux                                                                                                                                                                                              
    Arch: arm64                                                                                                                                                                                                  
    Version: #1 SMP Thu Jan 15 14:58:53 UTC 2026                                                                                                                                                               
    Available memory (MB): 19972
    Available CPU cores: 16
  Binaries:
    Node: 20.20.0
    npm: 10.8.2
    Yarn: 1.22.22
    pnpm: 10.29.2
  Relevant Packages:
    next: 16.2.0-canary.35 // Latest available version is detected (16.2.0-canary.35).
    eslint-config-next: N/A
    react: 19.2.4
    react-dom: 19.2.4
    typescript: 5.9.3
  Next.js Config:
    output: N/A

Which area(s) are affected? (Select all that apply)

Turbopack

Which stage(s) are affected? (Select all that apply)

Vercel (Deployed)

Additional context

No response