Multiple `export default` in `<script>` — JS throws, TS does not

[Fi2zz]

Version

3.5.24

Reproduction link

github.com

Steps to reproduce

1. Clone the repo: https://github.com/Fi2zz/vue-compiler-ts-multi-export-default
2. Install deps: yarn install (or npm install)
3. JS repro: npx tsx compileJavascriptCode.ts
   • Expected result: throws duplicate default export error
   • Actual result: SyntaxError: [vue/compiler-sfc] Only one default export allowed per module.
4. TS repro: npx tsx compileTypescriptCode.ts
   • Expected result: same error as JS (multiple default exports should be invalid)
   • Actual result: no error; logs include:
     • lang-> ts
     • content-> followed by two export default { ... } blocks

What is expected?

TypeScript and JavaScript should be consistent: when the same module contains multiple export default declarations, both should report an error.

What is actually happening?

• JS: throws DuplicateDefaultExport (Only one default export allowed per module).
• TS: no error; compileScript returns content keeping both default exports.

---

This repro uses vue/compiler-sfc with parse + compileScript.
Files of interest:

• src/ComponentVanilla.vue (JS)
• src/ComponentTS.vue (TS)
• compile.ts (driver calling parse/compileScript)

<!-- generated by vue-issues. DO NOT REMOVE -->

[edison1105]

In a regular <script> block, Vue runs the script through Babel. During its module transformation, Babel performs a safety check called checkDuplicateExports, which throws an error as soon as it finds a second export default.

When you switch to <script lang="ts">, the processing pipeline moves to TypeScript tooling—typically ts-loader, esbuild, or similar—and these setups are often configured in “transpile-only” mode to strip types without running full semantic checks. Because Babel no longer participates, its duplicate-export check never fires, so the checkDuplicateExports error goes away. The code is still invalid, though: the second default export simply overwrites the first, so you should fix the logic.

TypeScript also reports this explicitly with the error A module cannot have multiple default exports.ts(2528). see playground

Considering that this stems from how the source code is written rather than a Vue compiler bug, we won’t be making additional changes at the compiler level for now.